PURPOSE: To enable cooking with favorable result by selecting a constant to decide termination timing of cooking according to the power source frequency.

CONSTITUTION: When cooking is started, gas evaporates from food and this evaporated gas is detected by a gas sensor 24 and if output voltage from the gas sensor 24 changes by certain amount after reaching the peak value, a controller 20 takes in time required up to that time and sets a value issued by multiplying it by a constant as heating time after that. And when it becomes termination timing for the cooking, the controller 20 de-energizes a relay 25. When the relay is de-energized, a contact 25a becomes OFF and the oscillation of a magnetron 7 stops and the cooking finishes. Further, the constant is selected according to the cooking menu and power source frequency. Hereby, cooking with favorable result becomes possible.