PURPOSE: To realize stoppage at a definite position with high precision even in case of high-speed rotation of a rotary shaft by providing a cylinder part, not only concentric but also rotative in one body with the rotary shaft, and further providing a transmitter in a position on the inner periphery of said cylinder part.

CONSTITUTION: A magnet 7 is fitted in a position on the inner periphery of a cylinder part 13, which is formed in one body with a rotary shaft 6. The free end part of a fixed member 14 is inserted inside the cylinder part 13, while a magnet detector 8 is fitted to a side of said free end part. Since the magnet 7 is fitted in a position on the inner periphery of the cylinder part 13 formed in one body with the rotary shaft 6, even if big centrifugal force works on said magnet 7 due to high-speed rotation of the rotary shaft 6, said magnet 7 is pressed against the inner periphery of the cylinder part 13. Accordingly, centrifugal force working on a transmitter is received by the cylinder part itself, therefore, destruction and dispersion due to deterioration of the fitting part can be prevented beforehand.