PURPOSE: To detect the correct revolutions of a fan by computing the revolutions thereof from the time during which the fan rotates one cycle.

CONSTITUTION: A fan 1 has (n) poles. When a Hall element 2 detects revolutions of a fan 1 and outputs (n) pulses, namely, the fan 1 rotates one cycle, a control section 7 shifts data of a timer counter 9 to an arithmetic section 6 and resets the counter 9 again. The timer counter 9 counts pulses 10 of a higher frequency than the pulses from the element 2 as counted with a pulse counter 8. Data on a computing section 6 give the time required for one revolution of the fan 1. Taking the inverse number of the data thereon 6 enables the determination of the revolutions per sec of the fan 1.