PURPOSE: To suppress a sacrifice of a response speed, to prevent an influence exerted on an engine, of a pulsation of air, and to prevent the hunting by constituting a titled device so that an output of a heat wire sensor is smoothed, only when a pulsation is generated in air in a suction path.

CONSTITUTION: An air quantity in a suction path 7 pulsates in accordance with each process of suction, compression, explosion and exhaust in an engine body 8, when an opening of a throttle valve 3 exceeds a prescribed value and also it is within a prescribed range. A heat wire sensor 2 detects this air pulsation because its response speed is high. However, when the opening of the throttle valve 3 is smaller than the prescribed value, and its opening variation rate is outside the prescribed range, a pulsation of an output voltage V0 of the heat wire sensor 2 is a negligible degree. Therefore, a prescribed processing is executed by inputting an output of a throttle position sensor 18 to a controlling circuit 22, so that its output voltage is smoothed only in a state that the pulsation is such a large extent as cannot be disregarded.