PURPOSE: To prevent the softening of a heat-treated vegetable, by irradiating cut surface of a vegetable directly with far infrared ray under controlled temperature.

CONSTITUTION: A vegetable is cut to a proper size and shape according to the use. All the cut surface of the vegetable is perpendicularly and uniformly irradiated with a far infrared ray without carrying out heat-treatment for the inactivation of enzyme, cooking, etc. Any commercially available far infrared ray heater can be used for the purpose, however, a heater having high radiation efficiency is preferable. Although a part of the vegetable is dried by the irradiation treatment, the intensity of the far infrared ray has little influence on the softening-preventing effect of vegetable with heat. Accordingly, the far infrared ray irradiation can be carried out even in a vessel having controlled humidity or controlled at a low temperature. Since the weight decrease of the vegetable caused by the evaporation of water is dependent on the radiation intensity, the extent of the irradiation treatment is controlled based on the target weight decrease corresponding to the property of the vegetable.