PURPOSE: To surely detect a trouble when a fan is troubled by obtaining the OR of the output of a means to detect the stop of the fan and the output of a means to detect the cut-off of the primary current of the fan and outputting an alarm after the output of an OR means is protected for the prescribed time.

CONSTITUTION: After the output of an OR means 40 is protected for the prescribed time, a protecting means 50 to output an alarm obtains the OR of the output of a first trouble detecting means 20 to detect the stop of a fan 10 from the output of an alarm output circuit (a) and the output of a second trouble detecting means 30 to detect that the primary side current of the fan 10 cuts off, and after the protection is performed for the prescribed time by the protecting means 50, outputs the alarm. That is, when the fan 10 is stopped due to the foreign matters mixing, etc., the abnormality is detected by the first trouble detecting means 20, and when rotation is performed after stoppage due to the burning, etc., of the primary side coil of the fan 10, the second trouble detecting means 30 detects the abnormality. The OR of the output of first and second trouble detecting means 20 and 30 is obtained, outputted and the trouble of the fan 10 is detected.