The present invention makes it possible to inject a sterilization agent into a container in a state where there is no residual water in the object to be sterilized. The present invention involves: depressurizing to a pressure higher than the triple point pressure at which water freezes; then verifying the presence/absence of residual water in the object to be sterilized by comparing the actually measured depressurization time and the reference depressurization time, or comparing the actually measured pressure-increase rate or the reference pressure-increase rate; when residual water is present, returning the interior pressure of the container to atmospheric pressure or the vicinity thereof, and heating the object to be sterilized; then depressurizing again and discharging the residual water by vacuum boiling; repeating the residual-water discharge until there is no residual water remaining in the object to be sterilized; and then injecting the sterilization agent into the container.