An automatic tuning assist circuit is coupled in series with a transmission antenna. A first switch and a second switch are arranged in series between a first terminal and a second terminal of the automatic tuning assist circuit. Furthermore, a third switch and a fourth switch are arranged in series between the first terminal and the second terminal. A first auxiliary capacitor is arranged between a connection node that connects the first switch and the second switch and a connection node that connects the third switch and the fourth switch. A control unit switches the first switch through the fourth switch with the same frequency as that of the driving voltage, and with a predetermined phase difference with respect to the driving voltage.