To perform the steam sterilization of a product provided with a narrow inner hole or the like by performing constitution by the process of guiding the pressure of a sterilization chamber to the pressure for vaporizing a liquid sterilization agent from a non diffusion limit area and performing sterilization and the process for guiding it to the pressure for performing vaporization from a diffusion limit area where a minimum pressure is lower than the maximum pressure and performing the sterilization.

The diluted aqueous solution of hydrogen peroxide is put in a well 8, the pressure of the sterilization chamber 2 is reduced, the hydrogen peroxide is vaporized and a surface to be sterilized arranged at a metal grid 12 on a tray 14 is sterilized. At the time, this 2-stage method by different optimum pressures is adopted so as to sterilize the inside and outside of the product. That is, a first stage is executed by the low pressure range of about an atmospheric pressure and about 20Torr. A second stage is performed between about 1 and about 30Torr, preferably between about 5 and 10Torr. By the method, a non-accessible product and the product provided with a long and narrow inner hole are effectively sterilized.