A sterilization method for a medical instrument housed in a housing container that includes a container main body including a bottom portion at a lower end, a peripheral wall portion continuing to a periphery of the bottom portion and extending toward an upper end, and an opening surrounded by an upper end of the peripheral wall portion; and a gas permeable and micro-particle impermeable cover sheet that covers and seals the opening, the method comprising: a temperature-increasing step of introducing high pressure steam into an autoclave to increase a temperature within the autoclave to a prescribed temperature after housing the housing container having the cover sheet being thermally bonded to the upper end of the peripheral wall portion of the container main body housing the medical instrument with a heat seal thermoplastic resin to seal the opening in the autoclave; a sterilizing step of holding the temperature within the autoclave at the prescribed temperature for a prescribed time and performing sterilization on the medical instrument; and a cooling step of cooling a interior of the autoclave maintained at the prescribed temperature, characterized in that said medical instrument is housed in a plastic housing container that includes a plastic container main body; in said temperature-increasing step of introducing high pressure steam the high pressure stream is introduced intermittently into the autoclave; in each of 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 plastic housing container and inhibit the cover sheet from peeling from the upper end of the peripheral wall portion, wherein, during the temperature-increasing step, when the current pressure in the autoclave becomes lower than a temperature-increasing pressure, which is a pressure obtained by adding a preset pressure value to a saturated steam pressure corresponding to a current temperature in the autoclave, the compressed gas is introduced to increase the current pressure to higher than or equal to the temperature-increasing pressure, and wherein, during the sterilizing step, when the current pressure in the autoclave becomes lower than a sterilizing pressure, which is a pressure obtained by adding a preset pressure value to a saturated steam pressure corresponding to a current temperature in the autoclave, the compressed gas is introduced to increase the current pressure to higher than or equal to the sterilizing pressure. a temperature-increasing step of introducing high pressure steam into an autoclave to increase a temperature within the autoclave to a prescribed temperature after housing the housing container having the cover sheet being thermally bonded to the upper end of the peripheral wall portion of the container main body housing the medical instrument with a heat seal thermoplastic resin to seal the opening in the autoclave; a sterilizing step of holding the temperature within the autoclave at the prescribed temperature for a prescribed time and performing sterilization on the medical instrument; and a cooling step of cooling a interior of the autoclave maintained at the prescribed temperature,