Abstract: In an air conditioner control device, edges of change of the sensor outputs are detected by the second derivative of the outputs of WO3 gas sensor for NOx detection. The edge represents that the sensor is exposed to diesel exhaust. Its response to diesel exhaust is quick and it does not affected much by the influence of the wind. When the edge is detected, an intake of an automobile ventilator is dosed and the inflow of contaminated outdoor air is interrupted. An AD convertor stored micro computer is used for processing the sensor signal. In order to compensate the shortage of resolution of the AD convertor, the outputs are first differentiated analogically, then amplified and converted to digital signals, and finally further differentiated digitally.