Steam-air retort is added with a fan as a driving force to mix the steam and air, and to prevent cold spots in the machine. The heating medium and the processed food packages are in direct contact and in forced convective heating. In addition, the air is allowed to be introduced into the vessel during the sterilization. Moreover, the temperature and pressure can be controlled independently. This retort conducts several stages, which can be properly set and executed according to the type of the food package.