A contactless electricity-supplying device (20) includes a secondary winding (201) to which electric power is supplied from a primary winding (101), by an AC power supply (6). The impedance characteristic (Z) of Z1 in regard to the frequency has a local maximum (ZMAX) near the frequency (f0) of the fundamental wave component of aforementioned AC power supply (6); and the impedance characteristic (Z) of Z2 in regard to the frequency has the aforementioned frequency (f0) of the fundamental wave component to be between, a frequency (fMAX) that has its local maximum (ZMAX) nearest to aforementioned frequency (f0) of the fundamental wave component, and a frequency (fMIN) that has its local minimum (ZMin) nearest to the frequency (f0) of the fundamental wave component. Z1 indicates that the coupling coefficient (k) between aforementioned primary winding (101) and aforementioned secondary winding (201) is a prescribed value (0.3), and that it is an impedance of just the primary side (Z1) as seen from the output side of aforementioned AC power supply (6); and Z2 indicates that the coupling coefficient (k) between aforementioned primary winding (101) and aforementioned secondary winding (201) is the aforementioned prescribed value (0.3), and that it is an impedance of just the secondary side (Z2) as seen from the side of a load (72) to be connected to aforementioned secondary winding (201).