PURPOSE: To rationalize a filter replacing period by providing a filter replacement judging means wherein an electret filter replacing display is operated when the integrated value of the product of the degree of air contamination due to a gas sensor and the number of rotations of a fan exceeds a predetermined value.

CONSTITUTION: A charge part 1 consisting of discharge wires 2, parallel plate electrodes 3 and a DC voltage power supply 4 is provided on the upstream side with respect to an air stream and an electret filter 6 is provided on the downstream side. The filter 6 is arranged in a corrugated shape so as to cross an air passage and a gas sensor 7 detecting the contamination of air is provided on the upstream side of the filter 6 and a means 10 detecting the number of rotations of an air suction fan 8 is arranged on the downstream side of the filter 6. In a filter replacement judging means 11, the replacement display of the filter 6 is operated when the integrated value of the product of the degree of air contamination due to the gas sensor 7 and the number of rotations of the fan exceeds a predetermined value. As a result, the replacement period of the electret filter can be rationalized.