PURPOSE: To surely perform rotation speed detection by making output voltage generated at a sensor in accordance with rotation of a hub, higher.

CONSTITUTION: A sensor 20a consists of a permanent magnet 22a, a stator 23, and a coil 24. On the inner peripheral surface of the permanent magnet 22a, gear-like protruding and recessing parts are formed, and this inner peripheral surface is made to face the outer peripheral surface of a small diameter part 14 of a tone wheel 13 with a fine gap 25 in between. On a part of the small diameter part 14 that faces the inner peripheral surface of the permanent magnet 22a, multiple through holes 17 and 17 are, with a pitch equal to the protruding and recessing parts, formed in the circumferential direction at a constant interval. The inner peripheral surface of an inner cylindrical part 27 and a side surface of a ring part 34 which constitute the stator 23 face the outer peripheral surface of the small diameter part 14 of the tone wheel 13 and a side surface of a step 16, respectively, with the fine gap 25 in between. A facing area of the stator 23 and the tone wheel 13 is larger and magnetic reluctance between both the member 23 and 13 decreases, so that the output of the sensor 20a is increased.