PURPOSE: To detect the number of revolutions up to a high rotation, and also to improve the measurement accuracy of the number of revolution by packing a nonmetal to a slit part of a slit disk of a nonferrous metal.

CONSTITUTION: The number of revolutions is detected by making a slit disk 2 of a nonferrous metal which is attached to a revolving shaft 1 pass through between a transmitting coil 3 and a detecting coil 4. By packing a non-metal into a slit part 2a of this disk 2, a disk of a large diameter can be rotated at a high rotation, the number of revolutions can be detected up to the high rotation, and also the measurement accuracy of the revolutions is improved.