PURPOSE: To improve detection precision by about four times by obtaining one pulse per rotation of a revolving shaft as the output of the first comparing circuit and obtaining a square wave, which has a frequency four times as high as a frequency corresponding to the number of notched grooves of the second gear, and a square wave, which has a phase difference of 1/4 period from said square wave, as the output of a logic circuit.

CONSTITUTION: Outputs Z and Z' from a magnetic sensor 5 are supplied to a differential amplifier 813 as the first differential amplifying circuit, and a difference between them is obtained. The difference signal is compared with a reference voltage in a comparator 849 as the first comparing circuit, and one square wave is outputted through a driver 873 per rotation of the gear. Outputs A and A' from a magnetic sensor 6 are applied to a differential amplifier 811 as the second differential amplifying circuit, and a difference between them is obtained and is supplied to a vector synthetic comparing circuit consisting of comparators 841 and 845, vector synthetic comparators 842W844, and vector synthetic comparators 846W848. Outputs B and B' from the magnetic sensor 6 are applied to a differential amplifier 812 as the third differential amplifying circuit, and a difference between them is obtained and is supplied to the vector synthetic comparing circuit similarly.