Soybeans were boiled, innoculated with Bacillus natto IAM 1114, and incubated at 37°C for 24 hr. A protease to be produced by this microorganism was homogenized in a phosphate buffer, pH 7.0, and the supernatant submitted to (NH4) 2SO4 fractionation. A 30-80% saturation with respect to this salt was chromatographed on DEAE-Sephadex A-50, Sephadex G-100, and CM-Sephadex to obtain a partially purified protease fraction. Experiments with the purified protease showed that it had an optimal pH of 7.0 in hydrolysis of casein used as a substrate. Investigations with specific inhibitors and others indicated that this protease belonged to a subtilisin family. It is noted that the protease had an activity of effectively hydrolyzing a soy protein isolate used as another substrate.