A power supply apparatus for a sliding door includes a body-side fixture mounted on an automobile body, a door-side fixture mounted on the sliding door, and a flat cable gripped in the body-side fixture and the door-side fixture for supplying power to the sliding door. The flat cable is pulled out from the body-side fixture and/or the door-side fixture to form a convex flexural portion during opening and closing of the sliding door. The flat cable pull-out direction conforms with the protrusion direction of the convex flexural portion.