PURPOSE: To enable foreknowledge of the life of a blower before its function is brought to a stop, by a method wherein a time comparing circuit is added, and when a reducing time, in which the number of revolutions of the blower becomes below the predetermined number of revolutions after a power source is disconnected, is decreased over a reference reducing time, an alarm signal is outputted.

CONSTITUTION: A time measuring circuit 7 measures reducing times T1 and T2 between a time when a disconnecting signal (a), indicating disconnection of a power source 1 of a blower, is received from a power source signal 8 and a time when detecting signal (c) for reduction in the number of revolutions is fed out from a comparing circuit 5 for the number of revolutions, and outputs a reducing time signal (d) corresponding to the reducing times T1 and T2. When a reference reducing time TS, being set by a blower, which is brought to a condition in that the life is almost terminated due to the degree of deterioration of grease, and a time setting circuit 10, exceeds a reducing time T for which the number of revolutions of a blower 2 becomes below the predetermined number of revolutions after a power sourcce 11 is disconnected, a time comparing circuit 9 outputs an alarm signal (e), and an alarming circuit 12 warns a user of a trouble. If the user receives an alarm and replaces a blower, this constitution permits prevention of the occurrence of a trouble in which the function is brought to a stop during the use due to a trouble on the blower.