To obtain a high-quality cultured product such as koji with no concern for sundry germ contamination, by holding microorganisms and a substrate in a closed vessel and suppressing the temperature rise during culture process by promoting evaporation through pressure reduction to culture the microorganisms under simplified temperature control.

Microorganisms such as bacteria or yeast and a substrate are held in a closed vessel, the temperature rise during culture process is suppressed by promoting evaporation by pressure reduction and scavenging heat in the form of the latent heat involved in the evaporation. It is preferable to conduct the culture in such a way that the concentration of the carbon dioxide generated from the microorganisms is measured, and when the measurement result exceeds a specified value, the vessel is depressurized to decrease the amount of the carbon dioxide. Furthermore, it is preferable that the culture be conducted using an apparatus equipped with a control unit 11 to control the pressure reducing device based on the data from a temperature detector 8, a carbon dioxide detector 9 and a thermostat.