A first power transfer circuit in a first power transfer apparatus and a second power transfer circuit in a second power transfer apparatus are configured to be electrically symmetrical to each other. In transfer of electric power from one of the first and second power transfer apparatuses to the other of the first and second power transfer apparatuses, a controller drives a first switching circuit and a second switching circuit using a parallel resonant frequency of a complex resonant circuit as a reference frequency of an operating frequency. The controller changes the operating frequency to a frequency higher than the reference frequency or a frequency lower than the reference frequency if a duty ratio is maximized when transmitted electric power is lower than target electric power, which is a control target value, during the driving at the reference frequency.