What a kojimuro is
A kojimuro is a room for growing koji mould on steamed rice. A JETRO Oita explanation of sake making describes the koji-making stage as sprinkling seed koji on steamed rice in the kojimuro to multiply the mould. Because the mould needs steady warmth and humidity, the room is built as a closed chamber little affected by the outside air.
Walls and ceiling are usually thick for insulation and the entrance small. Such rooms are found in miso and soy sauce works as well as sake breweries. The koji is then used in making the yeast starter and the main mash.
Reading the records
Records note where the kojimuro is or how it is attached. The north brewery building of Gyokusendo in Yoro, Gifu, was once a brewing house with a kojimuro upstairs, where rice steamed in another building was spread out and its temperature controlled. The east brewery of Marumoto in Asakuchi, Okayama, two parallel buildings, has a former kojimuro attached to the north of the eastern one.
As brewing methods changed, many kojimuro were converted or removed; words such as former in the record point to earlier use. On site, a small closed room with few windows or a thick-walled corner may be the remains of one.
References
- 清酒の製造工程｜ジェトロ大分
- https://www.jetro.go.jp/jetro/japan/oita/industry/brewery/seisyu.html
- 玉泉堂酒造北酒蔵｜国指定文化財等データベース（文化庁）
- https://kunishitei.bunka.go.jp/heritage/detail/101/00009962
- 丸本酒造東蔵｜国指定文化財等データベース（文化庁）
- https://kunishitei.bunka.go.jp/heritage/detail/101/00003702
Last verified: 2026.09.26