In a power supply apparatus, one of a transformer and a first semiconductor device is stacked on the other thereof to constitute a stack assembly. The stack assembly, a second semiconductor device, and a choke coil are located on the major surface. A primary direct-current wire, which connects the first semiconductor device to a direct-current power source, is drawn out from a first predetermined portion of the stack assembly. A primary alternating-current wire, which connects the first semiconductor device and the transformer, is drawn out from a second predetermined portion of the stack assembly. The primary direct-current wire and the primary alternating-current wire are located to be separated from each other via at least a part of the stack assembly.