PURPOSE: To increase the precision of detection and keep the servo loop free from effects even when the rotating speed is changed, by taking out the magnetic flux detection output obtained by rotating the magnetic flux detector along with the rotation of the object being measured within the rotary magnetic field.

CONSTITUTION: When currents differing in phase by 90° each other are supplied from an oscillator 3 into coils 1, 2 which are disposed at approximately right angle, a rotary magnetic field is created in the coils 1, 2. In this rotary magnetic field is disposed a magnetic flux detector 4 which is attached to a rotary shaft 6 of the rotating body being detected 5. When the magnetic flux detector 4 rotates in the rotary magnetic field, the direction of the magnetic flux detector 4 and the rotary magnetic field direction are brought into a specified relation. At that moment, a plurality of positive and negative output pulses will be obtained, which are taken out by an output detector 7. By counting the number of pulses, the rotating body will be detected. When the rotating speed is changed, meanwhile, since the number of pulses detected within one rotation of the rotating body can be kept constant by changing the frequency of the rotary magnetic field, no effects may be applied to the servo loop.