Provided is a sliding power supply device which can reduce entire length of a case extendably housing a wiring harness, for a large slide stroke of a sliding seat and such. The sliding power supply device includes a long case extendably housing a wiring harness, a slider arranged slidable in a longitudinal direction of the case and arranged to lead out the wiring harness to outside of the case, and an extra length absorption portion arranged to support a wiring harness portion lead outside of the case such that the wiring harness portion is lead out at location further away from a movement end of the slider in the longitudinal direction of the case. The extra length absorption portion is a subcase having at least a bottom plate portion. Both ends of the wiring harness portion in a movement direction of the slider are positioned at the same height.