To instantaneously generate steam used only for sterilization by minimizing the radiation of the internal thermal energy from a sterilization chamber to the outside to keep the heart retaining property, and minimizing the consumption of the steam used for sterilization as much as possible.

The sterilization chamber 1 has a double structure consisting of an inner chamber and an outer chamber constituted so as to provide a space between the inner chamber and the outer chamber, and the air in the space is dischargeable. A heater 2 having a planar space is provided on the back surface side within the sterilization chamber 1.