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. Equivalent mutual inductances and equivalent mutual capacitances are formed through electromagnetic field resonant coupling between a primary winding and secondary windings, and a multi-resonant circuit including an LC resonant circuit formed in each of the primary side and the secondary side is formed. Electric power is transmitted from the primary side circuit to the secondary side circuit, and 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.