PURPOSE: To measure a speed with high accuracy even when a moving body is at an equal speed or accelerated or decelerated at its start or stop by measuring a frequency only when a signal obtained by rectifying and detecting a narrow-band irregular signal is higher than a threshold voltage.

CONSTITUTION: The narrow-band irregular signal outputted by an integrator 19 is branched through an LPF 20 for frequency measurement and signal decision. When the frequency is measured, a zero-cross pulse rectifying device 21 shapes the narrow-band irregular signal into a rectangular wave pulse, and a counter 23 counts the period of the signal and inputs the counted value to a microcomputer (μC) 31. Further, when a signal is decided, the signal from the LPF 20 is rectified 26 on a full-wave basis and detected 27, and then compared by a comparator 28 with a threshold voltage Vref. The output signal of the comparator 28 and the output signal of an FF 29 are inputted to an AND circuit 30, whose output signal is inputted to the μC 31. Then the μC 31 reads in data only when the output of the AND circuit 30 is at 'H' and clears a data latch circuit 24 and an FF 29.