PURPOSE: To enable to select the most suitable heating sequence, by discriminating existence of a cover for a material to be heated through the time length necessary until a detected quantity of gas, such as steam, alcohol or carbonic acid gas generated from a material to be heated reaches a fixed value.

CONSTITUTION: At the time of heating of a material 7 to be heatd in a heating chamber 6, the quantity of gas 12, such as steam, alcohol or carbonic acid gas to be generated from the material 7 to be heated is detected by either a humidity sensor or a gas sensor 10. Existence of a cover for the material to be heated is discriminated, additional heating time length is calculated and energization to a magnetron 8 is controlled by a control part 9 by measuring time necessary until said detected quantity reaches a fixed value. Then, indication is made on an indication part 5 and it is corrected by a keyboard 4 when the decision is wrong.