A contactless extension plug (3, 4) comprising: a first extension plug includes:) a first plug power reception coil; a first plug power transmission coil; and a first resonance capacitor connected in series between the first plug power reception coil and the first plug power transmission coil to configure a closed circuit, wherein the first resonance capacitor has a capacitance set to cancel secondary leakage inductance at the first plug power reception coil; characterized by at least one further, second extension plug, wherein: the second extension plug includes: a second plug power reception coil arranged facing the first power transmission coil; a second plug power transmission coil; and a second resonance capacitor connected in series between the second plug power reception coil and the second plug power transmission coil to configure a closed circuit, wherein the second resonance capacitor has a capacitance set to cancel secondary leakage inductance at the second plug power reception coil; the second plug power reception coil interlinks with an alternating magnetic field formed by the first power transmission coil to generate an induced electromotive force; the second plug power transmission coil forms an alternating magnetic field based on the induced electromotive force generated by the second plug power reception coil; the first plug power transmission coil and the second plug power reception coil are configured to satisfy a relational equation of L 2 / L 1 1 / 2 = G / K = N 2 / N 1, where L 1 represents a self-inductance of the first plug power transmission coil, L 2 represents a self-inductance of the second plug power reception coil, N 1 represents a winding number of the first plug power transmission coil, N 2 represents a winding number of the second plug power reception coil, K represents a coupling coefficient of the first plug power transmission coil and the second plug power reception coil, and G represents a voltage conversion gain of the voltage applied to the second plug power transmission coil relative to the voltage applied to the first plug power transmission coil; the first plug power transmission coil and the second plug power reception coil are identical with each other in shape, structure, and size but not in winding number; the voltage conversion gain G is set to one. a first extension plug includes:) a first plug power reception coil; a first plug power transmission coil; and a first resonance capacitor connected in series between the first plug power reception coil and the first plug power transmission coil to configure a closed circuit, wherein the first resonance capacitor has a capacitance set to cancel secondary leakage inductance at the first plug power reception coil;) a first plug power reception coil; a first plug power transmission coil; and a first resonance capacitor connected in series between the first plug power reception coil and the first plug power transmission coil to configure a closed circuit, wherein the first resonance capacitor has a capacitance set to cancel secondary leakage inductance at the first plug power reception coil; the second extension plug includes: a second plug power reception coil arranged facing the first power transmission coil; a second plug power transmission coil; and a second resonance capacitor connected in series between the second plug power reception coil and the second plug power transmission coil to configure a closed circuit, wherein the second resonance capacitor has a capacitance set to cancel secondary leakage inductance at the second plug power reception coil; a second plug power reception coil arranged facing the first power transmission coil; a second plug power transmission coil; and a second resonance capacitor connected in series between the second plug power reception coil and the second plug power transmission coil to configure a closed circuit, wherein the second resonance capacitor has a capacitance set to cancel secondary leakage inductance at the second plug power reception coil; the second plug power reception coil interlinks with an alternating magnetic field formed by the first power transmission coil to generate an induced electromotive force; the second plug power transmission coil forms an alternating magnetic field based on the induced electromotive force generated by the second plug power reception coil; the first plug power transmission coil and the second plug power reception coil are configured to satisfy a relational equation of L 2 / L 1 1 / 2 = G / K = N 2 / N 1, where L 1 represents a self-inductance of the first plug power transmission coil, L 2 represents a self-inductance of the second plug power reception coil, N 1 represents a winding number of the first plug power transmission coil, N 2 represents a winding number of the second plug power reception coil, K represents a coupling coefficient of the first plug power transmission coil and the second plug power reception coil, and G represents a voltage conversion gain of the voltage applied to the second plug power transmission coil relative to the voltage applied to the first plug power transmission coil; the first plug power transmission coil and the second plug power reception coil are identical with each other in shape, structure, and size but not in winding number; the voltage conversion gain G is set to one.