Abstract: The present invention relates to a koji production system capable of finely adjusting the water content of koji rice grains, comprising arranging a cooling and dehumidifying chamber and heating system 8 by means of a heat-pump process with a freezing cycle arranged in aerated circuit 5, in combination with heater H. In accordance with the present invention, koji can be produced at a large amount or at an extremely small amount, depending on the demand, and additionally, koji of various qualities such as high-quality koji for producing high-grade sake can be produced efficiently.