Provided is a power supply device for supplying power to a slide door via a wire harness, in which the wire harness is arranged invisible, which can improve bending durability of the wire harness with respect to sliding action of the slide door, and which can be used for the wire harness for multicircuit. In the power supply device, the wire harness is routed from a slide door arm through inside a slide door rail, through a roller and a guide plate and to a case. All the time during the operation of the slide door from a fully-opened state to a fully-closed state of the slide door, the wire harness is stored in a groove of the slide door rail or in the case.