PURPOSE: When food products of a solid-liquid mixture are heated by the irradiation of microwaves, an electrolyte is added to the liquid component of the food products to permit uniform heating without undesirable local heating.

CONSTITUTION: When fish, meat, vegetables, fruits or their cooked products containing solids and liquid are put in vessels made of low microwave dielectric loss, tightly sealed and they are heated by the irradiation of microwaves, an electrolyte such as sodium chloride is added to the liquid component of the food product by 0.03W2wt%.