A power supply apparatus (30) for sliding door includes a flat cable (4), a vehicle body side attachment (1) that supports the flat cable (4) in the vehicle body (19) side, and a door side attachment (2) that supports the flat cable (4) in the sliding door (20) side. The vehicle body side attachment (1) supports the flat cable (4) such that a longitudinal direction of the flat cable (4) has a predetermined first inclination angle relative to a horizontal plane. The door side attachment (2) supports the flat cable (4) such that the longitudinal direction of the flat cable (4) has a predetermined second inclination angle relative to the horizontal plane. The vehicle body side attachment (1) and the door side attachment (2) support the flat cable (4) such that a thickness direction of the flat cable (4) is substantially in parallel with the horizontal plane.