This experiment was carried out to investigate the producing conditions of organic acid and saccarifing amylase in Koji culture by Asp. usamii shirousamii $U_2$. The results were as follows. 1. When the strain $U_2$ was incubated at $30^{circ}C$, for 3 days in wheat flour and wheat bran media, the organic acid production was maximum. In the case of incubation at $35^{circ}C$, for 3 days in wheat flour medium and at $35^{circ}C$, for 2 days in wheat bran medium the activity of saccharifing amylase was highest. 2. When water was added 60% to wheat flour and 50% to wheat bran in the case of 3 days incubation, the organic acid production was superior. Both in wheat flour and wheat bran media, the saccharifing amylase production was most highly, when water was added 90-100%. 3. Comparatively speaking, the organic acid production was better in wheat flour medium than wheat bran medium, but the activity of saccharifing amylase was higher in wheat bran medium. 4. When the sweet potato starch waste and the wheat flour were mixed with same amount, the organic acid and saccharifing amylase production were higher than in simple wheat flour medium. 5. In the medium of sweet potato starch waste the organic acid and saccharifing amylase production were low extremely. 6. In the case of incubation at $30^{circ}C$, 3 days in wheat flour medium admixed with 60% water, the amount of citric acid in the organic acid formed was about 91%.