PURPOSE: To detect the number of persons with a high precision in a simple and inexpensive constitution in accordance with infrared rays emitted from human bodies by correcting the conversion from the width of the waveform of an output signal to the number of persons in accordance with a counted number of persons or a counted degree of congestion.

CONSTITUTION: Individual channels are synthesized as pulse trains having numbers of clock pulses, which are corresponding to pulse widths of outputs 13aW 13c of respective channels, in an output 20a of a multiplexer 20, that is, the output 20a is taken out as a pulse train corresponding to the measured number of persons. If it is necessary that this output 20a is displayed as a number, it passes a decade counter and is displayed on an LED by a driver. An analog switch 43 is normally close type; and when an output 42a is issued, the switch 43 is opened, and hereafter, the oscillation frequency of an astable multivibrator 21 becomes higher independently of a capacitor C2. That is, when the number of clock pulses (the measured number of persons) in the output 20a exceeds a prescribed value, the oscillation frequency of the astable multivibrator is made higher to correct the number of persons.