A power supply apparatus for a slidable structure which extends longitudinally, including: a pivotably-supported ring arm; a wiring harness having one end portion fixed to a tip portion of the ring arm and the other end portion fixed to a stationary structure; and an elastic member configured to bias the ring arm in a forward and upward direction. The ring arm is moved or rotated in a backward and downward direction while elastically deforming the elastic member due to tensile force of the wiring harness, when the slidable structure is moved forward toward its fully closed state. Also, the ring arm is moved or rotated in a forward and upward direction due to restoring force of the elastic member, when the slidable structure is moved backward toward its fully open state.