It is an object of the present invention to stably culture gas-utilizing microorganisms regardless of fluctuations in a supply flow rate of a substrate gas. The gas-utilizing microorganisms b are cultured in a liquid culture medium 9 that occupies a fermentative environment region 19 in a reaction tank 10. The gas-utilizing microorganisms b can be fermented in the fermentative environment region 19. A substrate gas is supplied to the fermentative environment region 19 from a gas supply member 22. A volume of the fermentative environment region 19 is controlled by a volume changing mechanism 30 according to the supply flow rate of the substrate gas.