PURPOSE: To prevent the practice of poor-finished heating in advance by announcing an error when the temperature of a heating chamber drops considerably due to the door opening and fails to exceed a certain temperature (lower than required temperature for preheating).

CONSTITUTION: When a door 3 is opened so as to house food in a heating chamber 2, a control section 7 shuts off power supply to a heater 6. When restart is indicated with a keyboard 4, the control section 7 performs affirmative consideration and a thermistor 9 inputs the in-chamber temperature detected. Then, the control section 7 compares the input detected temperature with a specific temperature which is lower than a required preheating temperature by k°C. When the opening time of the door 3 is long and hence the heated chamber especially preheated is cooled to a considerable extent, the detected temperature is lower than the specific temperature with regards to the above comparison. In this case, the control section 7 does not carry out heating operation and drives an error annunciator 10 for error warning. This construction makes it possible to prevent the operation of undesirable food heating under insufficient preheating conditions.