PURPOSE: To permit separate heating, in which grains, soup or the like on a tray for food supply are heated and raw vegetables or the like are not heated, by a method wherein a part for putting foods which require heating is provided in one direction, and another part for putting another food which does not require heating is provided in the other direction to heat selectively only the foods which require the heating.

CONSTITUTION: When foods on a tray for food supply are heated separately by employing a mircowave oven 13, the main body 1 of the tray is placed and arranged in the microwave oven 13 previously and a box type radiowave shielding wall 14, consisting of a microwave shielding material, is arranged on the side wall 2 of the main body 1 of the tray, then, microwaves are shielded by the radio wave shielding wall 14 whereby the foods 5, 5a... which does not require heating is not heated. On the other hand, the food 3, 3a..., which require heating, is heated and processed by the irradiation of microwaves and absorb the energy of electromagnetic waves whereby the food generates heat effectively by the conversion of heat.