PURPOSE: To enhance finish of cooking irrespective of the amount of food, by a method wherein the amount of food is detected through the rate of temperature change inside a heating chamber, and the output of a gas sensor is corrected, in a cooker wherein temperature is controlled by a gas sensor for detecting the gases generated from the food to be cooked.

CONSTITUTION: The relationship between heating time and exhaust gas temperature, i.e. the rate of temperature change, for each amount of food is prelimiarily stored in a controlling part 21, and the controlling part 21 detects the amount of the food 22 through the rate of temperature change inside the heating chamber 12 which rate is calculated from the temperature detected by a temperature detector 20 and is inputted into the part 21. The sensibility of the gas sensor 19 is corrected according to the amount of food thus detected. A magnetron 13 is controlled by turning ON or OFF a relay 17 by an output signal from the gas sensor 19 thus corrected. Accordingly, the food can be cooked to favorable finish irrespective of the amount of the food.