A preform is sterilized by a preform sterilizer. The preform is blow-molded by a blow-molding device to manufacture a container. Next, the container is filled with a culture medium by a filling device without sterilizing the container by a container sterilizer, and the container is plugged by a plugging device. Thereafter, it is verified how much microorganism survive or propagate in the culture medium in the container. A degree of sterilization in the container sterilizer is adjusted on the basis of a verification result.