PURPOSE: To detect a drop in the number of rotations by means of circuits built into a fan by comparing a voltage signal which is obtained by integrating a rotation signal of pulse width corresponding to the number of rotations of the fan with a threshold.

CONSTITUTION: A voltage signal which is obtained by integrating a rotation signal from a collector of a transistor Tr 51 by an integrating circuit 21 is compared 30 in a threshold test circuit 22 with threshold voltage which is set by resistors 31, 32, and an alarm signal corresponding to the result is output. That is, when a fan is normally rotating, a signal with constant pulse width is output to the collector of the Tr 51, and the circuit 21 outputs a voltage signal higher than the threshold voltage, so that the circuit 22 outputs an alarm signal of a low level, while if the number of rotations of the fan drops to approximately 30% or lower of the reference number of rotations, the voltage signal level is a threshold or lower, so that the alarm signal is at a high level. Thus a drop in the number of rotations can be detected by the circuits 21, 22 built into the fan, thereby detecting a failure in air-cooling operation prior to shutdown of a system.