PURPOSE: To prevent the generation of erroneous recognition, by providing a second pickup, to which iron powder and the like are not adsorbed, comparing the signal voltages of the first and the second pickups, thereby detecting the fault of the first pickup.

CONSTITUTION: At a gear 3 to be measured, a first pickup 7 and a second pickup 8 which uses a coil 6 as a magnetomotive force source are arranged. A rotary speed measuring device 9 and a fault signal generator 10 are connected. A reference for the difference between the values of signal voltages to be compared is set at a specified value. A current is conducted to the second pickup 8 from the fault signal generator 10 for a very short time. The signal voltages generated in the first and second voltages 7 and 8 are compared. When the difference becomes the specified value or more, the fault signal of the first pickup 7 is transmitted. In this case, since a magnetomotive force is not always present in the second pickup 8, iron powder and the like are not attached, thereby preventing erroneous recognition.