PURPOSE: To obtain high-grade puffed dry koji for brewing use, preservable for long period, and to provide a method for producing the puffed dry koji requiring no plant investment for maintaining frozen state/high vacuum and high pressure and low in running cost.

CONSTITUTION: Stock koji containing water of semi-bound water region is put in a carbon dioxide atmosphere while maintaining it at about ice temperature under normal pressures to incorporate the carbon dioxide into the semi- bound water through infiltration, dissolution and saturation. The resultant stock koji is then subjected to restricted heating under a reduced pressure of 10-160torr by microwave dielectric heating to produce voids and capillaries in the koji through the swelling pressure and running away of a moist heat gas generated and to accomplish a puffed drying of the koji to a moisture activity of ≤0.76, thus obtaining the objective puffed dry koji.