PURPOSE: To simply and efficiently carry out the continuous liquid culturing of a mold excellent in productivity by inoculating the mold having the protease productivity into a specific liquid culture medium and culturing the mold while keeping the concentration, etc., of dissolved oxygen during the culturing constant.

CONSTITUTION: A mold such as a filamentous fungus, having the protease productivity and belonging to the genus Aspergillus is inoculated into two or more culture media having substantially the same ratio (C/N ratio) of carbon to nitrogen as a liquid culture medium and cultured while keeping the concentration of dissolved oxygen or pH during the culturing constant. Thereby, the continuous liquid culturing of the mold is carried out.