A wireless power transmission system for transmitting power over a resonant magnetic field, the system comprising a power-transmitting resonator and a power-receiving resonator, wherein at least one of the power-transmitting resonator and the power-receiving resonator is a series resonant circuit, and wherein the series resonant circuit includes: an inductor including spiral wiring and first (213; 213a), second (207a, 207b, 207c) and third (207a, 207b, 207c) extended wires, the first extended wire for connecting a first connection point (203; 203a) of the spiral wiring to a power supplying structure, the second extended wire for connecting a second connection point (205a, 205b, 205c) of the spiral wiring, which is different from the first connection point, to the power supplying structure, the third extended wire for connecting a third connection point (205a, 205b, 205c) of the spiral wiring, which is more distant from the first connection point than the second connection point is, to the power supplying structure; multiple switches (211a, 211b, 211c), two of which are connected to the second and third extended wires, respectively; and multiple capacitors (209a, 209b, 209c), each of which is connected in series to a corresponding one of the second and third extended wires by way of a corresponding one of the switches, wherein the series resonant circuit has an inductance configured to be varied according to which current path has been chosen by selectively turning ON one of the switches, and characterized in that; the capacitances of the capacitors are configured so that the series resonant circuit has a substantially same resonant frequency irrespective of what switch has been turned ON selectively, and wherein the spiral wiring has a low-resistance portion, which is located in a range from the second connection point through the first connection point and of which the wiring resistance per unit length at the resonant frequency is set to be lower than in at least a part of the other range (201b, 201c) of the spiral wiring. an inductor including spiral wiring and first (213; 213a), second (207a, 207b, 207c) and third (207a, 207b, 207c) extended wires, the first extended wire for connecting a first connection point (203; 203a) of the spiral wiring to a power supplying structure, the second extended wire for connecting a second connection point (205a, 205b, 205c) of the spiral wiring, which is different from the first connection point, to the power supplying structure, the third extended wire for connecting a third connection point (205a, 205b, 205c) of the spiral wiring, which is more distant from the first connection point than the second connection point is, to the power supplying structure; multiple switches (211a, 211b, 211c), two of which are connected to the second and third extended wires, respectively; and multiple capacitors (209a, 209b, 209c), each of which is connected in series to a corresponding one of the second and third extended wires by way of a corresponding one of the switches, wherein the series resonant circuit has an inductance configured to be varied according to which current path has been chosen by selectively turning ON one of the switches, and characterized in that; the capacitances of the capacitors are configured so that the series resonant circuit has a substantially same resonant frequency irrespective of what switch has been turned ON selectively, and wherein the spiral wiring has a low-resistance portion, which is located in a range from the second connection point through the first connection point and of which the wiring resistance per unit length at the resonant frequency is set to be lower than in at least a part of the other range (201b, 201c) of the spiral wiring.