According to the present invention, a sterilant can be injected into a container without residual water on an object to be sterilized. The method includes decreasing a pressure in the container to a pressure higher than the triple point pressure at which water freezes. The method further includes comparing an actual time required for decreasing the pressure and a reference time, or comparing an actual rate of pressure increase and a reference rate to detect residual water on the object. If the residual water is detected, the pressure in the container is increased to the atmospheric pressure or the quasi-atmospheric pressure and the object is heated, and then the pressure is decreased to drain the residual water through the decompression boiling. The step of draining the residual water is repeatedly performed until no residual water is detected on the object and subsequently the sterilant is injected into the container.
