Colonies growing on a culture medium surface may be more accurately counted with reduced variation in the counts obtained by capturing a plurality of images of the entire culture medium surface, wherein successive images are captured following rotation of the culture medium surface relative to the image capture apparatus by n/360 degrees where n is the number of images to be captured; performing data processing on image data of the entire culture medium surface of the petri dish at each specified angle; and the number of colonies in the petri dish is calculated by performing numerical processing on the number of colonies counted separately at each specified angle.