An enclosed air sound sensor is fixed to a driver seat in order to capture a signal indicative of the movement of body of a driver and a duration where the signal does not reach a predetermined level is detected. At the same time, a signal being generated from a vehicle only during travel is captured and both signals are compared by an operating circuit. When both signals exist and only when a specific trend appears in the duration of the signal indicative of the movement of body, an alarm is generated. An inexpensive and easy-to-fix doze operation alarm operating only during travel of a vehicle is provided.