We studied the effect of proteolysis promotion of grain koji in mice. Grain koji is an enzyme-rich food which is produced by solid-state fermentation with Aspergillus oryzae in a mixture of six kinds of grains (foxtail millet, Japanese barnyard millet, millet, sorghum, barley, purple-black rice). Grain koji contains higher acid-protease and neutral-protease activities than unfermented grain materials. Therefore, grain koji could digest casein in the simulated gastric juice, giving rise to higher amounts of free amino acids. When male ICR mice were fed with CE-2 diets containing 10% grain koji for 28 days, a significantly (p<0.05) higher absorption ratio was observed in comparison with that in the control group of mice reared on CE-2 diets supplemented with 10% grain materials. The serum lipid concentrations were almost the same among the various mice groups. The present study has thus clarified the proteolysis promotion of grain koji.