Here, the center electrode 311 is constituted of a platy conductive member (for example, a member of copper, aluminum, or the like) having a circular shape with a radius r1. The annular electrode 312 is constituted of a platy conductive member having an annular shape with an outer radius r2 and a width w. Note that the center electrode 311 and the annular electrode 312 are disposed on a same plane, and the distance between an outer periphery of the center electrode 311 and an inner periphery of the annular electrode 312 is d1. One end of the inductor 313 is connected to the center electrode 311, and the other end is connected to one end of the connection line 315. One end of the inductor 314 is connected to the annular electrode 312, and the other end is connected to one end of the connection line 316. The connection lines 315, 316 are, for example, constituted of a coaxial cable or a balanced cable. Other ends of the connection lines 315, 316 are connected respectively to output terminals of a not-illustrated alternating-current power generating unit. By connecting the alternating-current power generating unit to the power transmission coupler 310 by the connection lines 315, 316, a power transmission device is constituted. Note that a resonance frequency of a series resonance circuit due to capacitance C of a capacitor formed by the center electrode 311 and the annular electrode 312 and inductance L of the inductors 313, 314 is f_c.