PURPOSE: To produce a large amount of high-quality fermented soybeans free from unevenness of qualities by packing boiled or steamed soybeams into packaging sheets or packaging bags capable of permeating oxygen while maintaining temperature and humidity required for fermentation and aging of fermented soybeans and fermenting.

CONSTITUTION: A main body 5 of a container having a great number of air passageways 9 capable of permeating oxygen is provided with a packaging sheets 7 or packaging bags (6: water absorbing paper at the outside of packaging sheet) having excellent heat retaining properties and a great number of air permeable holes 10 openable and closable corresponding to change of pressure at the interior and exterior, boiled or steamed soybeans 8 inoculated with Bacillus natto are packed into the packaging sheets 7 or packaging bags and the packaged boiled or steamed soybeans 8 together with the main body 5 of container are put in a fermenting chamber and fermented and aged.