Kasho Ship Flag〈April 28th, Tensho 9〉/Noshima Murakami

Kasho Ship Flag & Noshima Murakami Family Documents: The Passport of Japan's Greatest Pirates

In the waters of Japan's Seto Inland Sea, a powerful clan once ruled the waves. The Noshima Murakami, described by the 16th-century Portuguese missionary Luís Fróis as "Japan's greatest pirates," commanded the treacherous currents and narrow straits between the islands of what is now Ehime and Hiroshima prefectures. Today, a silk flag and a collection of over 200 historic letters preserve the living memory of these lords of the sea. The Kasho Ship Flag (dated Tensho 9, April 28 / 1581) and the Noshima Murakami Family Documents are designated as an Important Cultural Property of Japan, housed at the Yamaguchi Prefectural Archives.

What Is the Kasho Ship Flag?

The Kasho Ship Flag (過所船旗, kasho senki) was a type of naval transit pass — a flag that functioned as a certificate of safe passage through waters controlled by the Murakami clan. Ships that paid the hobechisen (帆別銭), a toll based on sail size, received this flag. By hoisting it on their mast, vessels were guaranteed protection from piracy and safe navigation through the perilous currents of the Seto Inland Sea.

This particular flag, made of plain silk (hira-ginu), measures approximately 53.4 cm in height and 42.1 cm in width. A wooden crossbar (hassō) and roller rod (jiku-gi) made of cedar remain attached. At the center, the stylized Murakami family crest — the character "上" (ue, meaning "above") — is boldly brushed in ink. In the lower-right corner, the recipient is identified as "Geishū Itsukushima, Hafuri-shi" (a Shinto priest of Itsukushima Shrine in Aki Province). In the lower-left, the date "Tensho 9, April 28" (1581) and the name and cipher (kaō) of Murakami Takeyoshi, lord of the Noshima Murakami clan, are inscribed.

Only two examples of Murakami Takeyoshi's kasho ship flags are known to survive in the entire country, making this artifact extraordinarily rare. The other is preserved at the Wakayama Prefectural Museum, addressed to the Mukai clan of Saika in Kii Province.

The Noshima Murakami Family Documents

Accompanying the flag is a remarkable collection of 203 historical documents (originally designated as 199 with 4 added later) that trace the political alliances, military campaigns, and diplomatic activities of the Noshima Murakami clan from the Muromachi period through the Azuchi-Momoyama period. These documents were passed down through the Murakami family, who served as the Chōshū Domain's Funategumi (naval unit) during the Edo period.

The collection includes correspondence from an astonishing range of historical figures: Ashikaga Yoshiteru (the Muromachi Shogun), Oda Nobunaga, Toyotomi Hideyoshi's brother Toyotomi Hidenaga, Mōri Motonari, Mōri Terumoto, Kobayakawa Takakage, Matsunaga Hisahide, Ōtomo Sōrin, Ōuchi Yoshinaga, and Kōno Michinao — warlords from across western Japan and the Kinai region. Nine of the documents are assembled into handscroll format (makimono), while 106 are preserved as individual sheets.

The earliest document in the collection is a letter from Hosokawa Takakuni appointing Murakami Takakatsu as governor of the Shiwaku Islands in Sanuki Province, establishing the eastern frontier of the Noshima Murakami's maritime domain. The most recent is a letter from Mōri Terumoto to Murakami Motoyoshi dated 1599, near the end of an era.

Why Were They Designated an Important Cultural Property?

The Kasho Ship Flag and Noshima Murakami Family Documents were designated as an Important Cultural Property (Jūyō Bunkazai) on September 4, 2015, with additional items added in 2017. The designation recognized several factors of outstanding historical value:

• The kasho flag is an exceptionally rare physical artifact — one of only two known surviving examples — that provides direct, tangible evidence of the maritime toll and transit-pass system that sustained the Murakami Kaizoku's naval economy.
• The document collection is the most comprehensive surviving archive related to the activities of Sengoku-period naval forces (suigun) in the Seto Inland Sea, offering unparalleled insight into medieval Japanese maritime governance.
• Together, they vividly illustrate the transformation of the Noshima Murakami from an independent maritime power to vassals of the Mōri clan, particularly in response to Toyotomi Hideyoshi's Anti-Piracy Edict (Kaizoku Kinshirei) of 1588.
• The correspondence with major feudal lords across western Japan, the Kinai region, Kyūshū, and Shikoku demonstrates the far-reaching political influence of the Noshima Murakami as key players in Sengoku-era diplomacy and warfare.

The Murakami Kaizoku: Lords of the Sea

To understand these artifacts, one must understand the remarkable Murakami clan. The Murakami Kaizoku (海賊, literally "sea raiders") were not mere pirates in the Western sense. They were a sophisticated maritime polity that controlled navigation, trade, and security across the Seto Inland Sea from the mid-14th century onward.

The clan consisted of three branches — Noshima, Kurushima, and Innoshima — collectively known as the "Santō Murakami." The Noshima branch, based on the tiny island of Noshima off the coast of modern-day Imabari in Ehime Prefecture, was considered the senior lineage. From their island fortress, they mastered the powerful tidal currents that could reach speeds of up to 10 knots, turning the treacherous waters into a natural defensive barrier.

Murakami Takeyoshi (1533–1604), who issued the kasho flag in this collection, was arguably the most famous leader of the Noshima branch. Despite the aggressive image associated with the word "pirate," Takeyoshi was known as a man of considerable culture. He regularly organized renga (linked poetry) gatherings at Ōyamazumi Shrine and was accomplished in the arts of incense and tea ceremony.

Highlights and Points of Interest

The Kasho Ship Flag offers a rare opportunity to see a medieval Japanese maritime document in its original physical form — silk, ink, and wood, preserved for over 440 years. The bold calligraphy of the "上" character crest at the center of the flag is both an artistic creation and a symbol of authority that once guaranteed safe passage through some of the most dangerous waters in East Asia.

The Noshima Murakami Family Documents, meanwhile, read like a who's who of Sengoku-era Japan. Letters bearing the names of Oda Nobunaga, the Mōri clan, and other legendary figures offer a window into the high-stakes diplomacy of the Warring States period, as seen through the unique perspective of a naval power.

Visitors interested in these artifacts can explore the wider world of the Murakami Kaizoku through the Murakami Kaizoku Museum on Ōshima Island, where replicas and related exhibits bring this maritime history to life. Boat tours to the ruins of Noshima Castle — the actual island fortress of the clan — offer an unforgettable experience of the tidal currents that defined their world.

Surrounding Area and Access

The Kasho Ship Flag and Noshima Murakami Family Documents are preserved at the Yamaguchi Prefectural Archives (山口県文書館) in Yamaguchi City, located within the Yamaguchi Prefectural Library building. As a deposited Important Cultural Property, viewing opportunities for the original artifacts may be limited to special exhibitions — it is advisable to contact the archives in advance.

For those wishing to immerse themselves in the world of the Murakami Kaizoku, the Murakami Kaizoku Museum on Ōshima Island in Imabari City, Ehime Prefecture, is the premier destination. Located along the famous Shimanami Kaido cycling route, the museum exhibits a wide collection of Noshima Murakami family treasures, armor, weapons, and cultural artifacts. From the museum pier, visitors can join boat tours to the Noshima Castle Ruins, a nationally designated historic site on an uninhabited island, and experience the powerful tidal currents firsthand.

Nearby attractions include Ōyamazumi Shrine on Ōmishima Island, one of Japan's oldest and most revered shrines, which houses an extraordinary collection of samurai armor and weapons. The Kirosan Observatory on Ōshima offers panoramic views of the islands where eight Murakami pirate castles once stood.