PURPOSE: To reliably detect the speed of a rotor by arranging a detection terminal having a coil which is excited at high frequency opposite the tooth part of a rotary gear closely and connecting an electric signal output terminal from the detection terminal to a measuring circuit.

CONSTITUTION: The coil 2 which is excited at the high frequency is arranged at the radial position of the tooth part 1a of the rotary gear 1. Then, an eddy current is generated at the gear 1 by the excitation of the coil 2 as shown in a figure (b) and this eddy current produces magnetic flux B in the opposite direction from magnetic flux A produced by the coil 2. Then the eddy current varies in synchronism with the intermittence of the tooth part 1a, so the self- inductance of the coil 2 varies. This appears depending upon the rotating speed of the gear 1, so a voltage signal as the output signal of the coil 2 is measured to detect the rotating speed of the gear 1.