To easily set a reference position of an electric motor based on a defined element, when a battery and the electric motor are connected to each other, without adding a component such as a detecting device.

The electric motor 4 is reversely rotated when the battery and the electric motor 4 are connected to each other, so as to drive a piston 5 to a state where a brake pad 2 separates from a brake rotor 1. A rise in the electric current of the electric motor 4 is acquired, thereby a driving end of the piston 5 is recognized. The value detected by a rotation angle sensor 9 is initialized by using the recognized driving end as the reference position of the piston 5.