A microwave oven includes a gas sensor for detecting the concentration of gas developed from a foodstuff disposed in an oven cavity. When the cooking operation is conducted to a desired level, the gas concentration reaches a preselected level and, hence, an output signal of the gas sensor shows a predetermined output level at which the microwave generation is terminated. An abnormal condition detection circuit is provided for detecting an abnormally high resistance created in the gas sensor due to, for example, disconnection of a warming heater included in the gas sensor. When an abnormal condition is detected by the abnormal condition detection circuit, the microwave generation is terminated and an alarm display is conducted to inform the operator of the abnormal condition.