PURPOSE: To suppress deterioration in freshness at a low value, by homogeneously heat-treating fishes, shellfishes or cattle meat at a specific temperature in a short time and inactivating enzymes.

CONSTITUTION: Heat treatment is homogeneously carried out with far infrared rays and microwaves in a very short time at a temperature, e.g., 40W70°C, for inactivating enzymes which are objects. The heat treatment is economically performed in warm water containing salt added thereto at a suitable temperature according to the objects for a prescribed time. In both cases, cooling treatment is carried out just after the heat treatment.