To provide an air cleaner capable of stable operation without frequent change in the number of rotation of a blower and also capable of quickly coping with the increase of a dust concentration in the air.

The cleaner is constituted so that the moving average value of the dust concentration detected by an optical dust sensor is determined and the number of rotation of the blower is decided by comparing the moving average value with a dirt standard value, and moreover, since an increase/decrease judging means for judging wether the output of the optical dust sensor is increasing or decreasing is provided and the moving average value is calculated by decreasing a number of a sample when the output of the optical dust sensor is increasing and by increasing the number of the sample when the output is decreasing, the cleaner is possible to quickly cope with the dirt of the air.