A reception power supplying unit generates each of reception signals before and after a switching operation. A signal processing circuit generates each of difference signals before and after the switching operation on the basis of the reception signal and a reference signal. A phase difference detector calculates, as a transmission phase difference, the phase difference between transmission power supplying units on the basis of the respective difference signals, and adjusts a phase shift amount on the basis of the transmission phase difference and a set phase difference that is previously set.