PURPOSE: To fix a magnet exclusively used for a sensor to the iron core of a rotor without using an adhesive when attaching the magnet exclusively used for a sensor to the iron core of a rotor.

CONSTITUTION: This device is equipped with a resinous disc 5, which has a hub with a center hole for the rotary shaft 2 of a rotor to pass through and a plurality of projections, and a cylindrical magnet 4 exclusively used for a magnet, which has a bottom being provided with a center hole far the hub of its resinous disc 5 to engage with and a hole or a recess for a plurality of projections to engage with when the hub of the resinous disc 5 engages with the center hole. And, a rotary shaft 2, where the iron core 1 of the rotor is fixed, press-fit in the center hole of the center holes of the sensor magnet 4 and the resinous disc 5 being united by fitting a plurality of projections of the resinous disc 5 into a plurality of holes of the sensor magnets 4 and at the same time, setting the hub of the resinous disc 5 into the center hole of the sensor magnet 4, thus the sensor magnet 4 is stuck fast to the iron core 1 of the rotor.