PURPOSE: To always and accurately determine the replacement time of a dust collecting filter according to the use conditions and the use environment without separately providing a sensor.

CONSTITUTION: Air in a room is sucked by a fan device 14 installed in a case body to discharge it through a filter part 15. Dust in the air is energized by the filter part 15 and is captured by an electrostatic filter 19. By a forked detecting flow passage 21 having a detecting flow passage 24 on the in-room side and a detecting flow passage 25 on the filter side, the air in the room is introduced from before and after the electrostatic filter 19 into a dust sensor 23 to detect the quantity of the dust. When the detected quantity of the dust in the air in the room reaches the judgement operation level or more, the detection flow passage 21 is switched by a flow passage switching mechanism 22 to detect the quantity of the dust, and the dust collecting capacity of the electrostatic filter 19 is judged to be lowered. The actual dust collecting capacity is accurately determined without separately providing a sensor.