A highly versatile method of a sterilization and a sterilization apparatus, in which objects with poor heat resistance that are to be sterilized, such as plastic products, can be sterilized without incurring thermal damage, with such a strong sterilization power that spores can be killed. A treatment chamber is filled with a treatment gas containing HCHO, and any sterilization objects placed in the treatment chamber are sterilized. During sterilization, while maintaining the interior of the treatment chamber at a pre-determined temperature (T1), the treatment gas is fed into the treatment chamber, and the sterilization objects are exposed to the treatment gas for a pre-determined period of time (h1). Thereafter, the feeding of treatment gas into the treatment chamber is discontinued, and while the treatment gas remains in the treatment chamber, the interior of the treatment chamber is heated to a heat treatment temperature (T2) that is higher than the pre-determined temperature (T1) but does not exceed the heat-resistance temperature (T0) of the objects to be sterilized.