PURPOSE: To facilitate a preheating time setting operation by a method wherein a preheating time corresponding to a set temperature is defined for every cooking menu and the preheating operation is terminated after the preheating time is elapsed.

CONSTITUTION: When a menu key 5-1 at the operating part 9 is operated, a cooking operation is started and also a preheating operation is started. As each of the cooking menus, a predetermined time and a cooking temperature are stored in a microcomputer 11, so that the microcomputer 11 calculates a maximum preheating time (t) corresponding to the predetermined time in accordance with the predetermined formula and the preheating time is set. The preheating operation is finished when the temperature 6 in the heating chamber is reached to the predetermined temperature or when the calculated time is elapsed. With this arrangement, the preheating operation can be simplified.