A power supply apparatus includes a transformer including primary and secondary coils, a heatsink that radiates heat, and a substrate supporting the heatsink and including primary and secondary circuits. The transformer outputs a voltage to the secondary coil according to a voltage input to the primary coil. The heatsink includes a protruding portion protruding from a position where the heatsink is in contact with the substrate in a direction protruding from a surface of the substrate. The heatsink further includes an extending portion extending from part of the protruding portion to an area of the secondary circuit of the transformer, in a state spaced apart from the substrate. The extending portion enters the area of the secondary circuit when viewed in a direction perpendicular to the surface of the substrate. A distance between the extending portion and the surface of the substrate is longer than at least 1 millimeter (mm).