A power supply device for a sliding door comprising an extra-length absorbing unit, a door side fixing unit fixed to a sliding door, a flexible tube extending from said extra-length absorbing unit to said door side fixing unit, and a wire harness wired from a vehicle body through said extra-length absorbing unit, said flexible tube and said door side fixing unit to the sliding door, wherein said extra-length absorbing unit includes a case having a gate for said flexible tube, a rotating drum for winding said flexible tube in the case, and a torsion spring for providing a turning force with said rotating drum in a winding direction of said flexible tube, an end portion at the vehicle body of said flexible tube being fixed on the rotating drum, a portion at the vehicle side of said wire harness being fixed to the rotating drum in a vicinity of the end portion of said flexible tube, said flexible tube being wound in spiral within the rotating drum, a spiral inner peripheral portion thereof being fixed to a center portion of the case, and said flexible tube being pulled out of the case through the gate, said door side fixing unit holds a sliding door side end portion of said flexible tube, a sliding door side portion of said wire harness being pulled out of the end portion of the flexible tube into the sliding door, and said extra-length absorbing unit is fixed to the vehicle body so as to be located in an intermediate position of a moving range of the door side fixing unit by an opening and closing movements of the sliding door, said flexible tube being provided with a tensile force by the turning force of the rotating drum from the extra-length absorbing unit to said door side fixing unit.