A branch wire branching from a trunk wire of a floor harness routed to an automobile is used as a wire harness supplying power to the sliding seat. The wire harness is inserted within an extra length accommodation case, and in addition, the extra length accommodation case is fixated, facing downward, to a seat rail to which the sliding seat is slidably mounted, and is accommodated in a space below the seat rail. The wire harness U-turned within the extra length accommodation case is passed through the slider and extracted to a sliding seat side and connected to the seat harness within the sliding seat. A bottom portion of the slider is slidably mounted to a straight line direction slider sliding groove provided on a top end of the extra length accommodation case and a top portion of the slider is fixated to the sliding seat.