A primary side resonant circuit is formed by a primary side resonant inductor and a primary side resonant capacitor, and secondary side resonant circuits are formed by secondary side resonant inductors and secondary side resonant capacitors. Mutual inductances are formed equivalently through magnetic field resonant coupling between a primary winding and secondary windings, and a multi-resonant circuit that includes two or more LC resonant circuits is formed by a primary side circuit and a secondary side circuit. Electric power is transmitted from the primary side circuit to the secondary side circuit; resonant energy that is not transmitted from the primary winding and, of energy which the secondary winding has received, resonant energy that is not supplied to an output are each retained in the multi-resonant circuit; and, at the secondary side, the resonant energy is retained in a current path in which a rectifying element is not formed in series.