To smoothly move a caliper, and to keep a contact face of a pat part and a disc in parallel at all times, by locating an axis point of a floating pin on a specific position, relating to that of a push-rod to be inserted into the caliper.

An electromagnetic part 13 has a shape of disc, so that an upper and lower pair of projection arm mounting parts 16, 16 are prepared by forming the semi-circular recessed parts 18, corresponding to the semi-circle of the electromagnetic part, and holding an unit 14 comprising an electromagnetic part and a cahiper compactly. An axis of the electromagnetic part 13 and that of a push-rod are agreed with each other, and the axis points Of, Of of the floating pins 17, 17 are located on a position about 180° symmetrical to the axis point O of the electromagnet 13 when observed from the axial direction of the electromagnetic part and the push-rod. That is, three axis points Of, O, Of are located on a straight line. Thereby the unit 14 can jog along the floating pins 17, 17 smoothly without the generation of offset.