PURPOSE: To enable reliably detection of clogging of a filter, by a method wherein a filter is located at a suction port side of an air passage in an air conditioner body, and a propeller fan type detector for the number of revolutions is mounted in the air passage.

CONSTITUTION: A filter 9 is mounted to the front of a heat exchanger 6 of an air conditioner body 1, and a propeller fan type detector 10 for the number of revolutions is positioned in an air passage 5. With a fan 8 started to operate when the filter 9 is clean, a suction air is discharged to the outside by the fan 8 through the running of a propeller fan 11. When the filter 9 is clogged and a quantity of the air reduces with the progress of an operation for a long time, a Hall element 16, of which a magnetic flux varies through rotation of a magnet 15 by the rotation of a propeller shaft 12, is reduced in an output. In which case, the output of the initial element 16 is stored in a microcomputer and the like, and the comparision of the initial output with the reduced output of the element 16 permits the detection for the contamination of the filter 9.