A sterilization method for a medical instrument housed in a housing container includes a temperature-increasing step of housing the housing container in the autoclave and introducing high pressure steam into the autoclave to increase a temperature within the autoclave to a prescribed temperature, a cover sheet being thermally bonded to a peripheral wall portion of the container main body a sterilizing step of holding the temperature within the autoclave at the prescribed temperature for a prescribed time to sterilize a medical instrument and a cooling step of reducing the temperature within the autoclave. In the temperature-increasing step, the sterilizing step, and the cooling step, compressed gas is introduced into the autoclave under a steam atmosphere obtained by the introduction of the high pressure steam to increase a current pressure in the autoclave to higher than or equal to a pressure in the housing container and inhibit the cover sheet from peeling from the peripheral wall portion.
