A frequency control circuit (45) controls an oscillation frequency of a second high frequency power source 51 based on a phase difference between a voltage component and a current component measured by a phase difference sensor (41) and an input impedance to an impedance matching device (34) measured by an impedance sensor (42). An amplitude control circuit (44) controls a level of a high frequency electricity output by the second high frequency power source (51) based on an electricity (effective electricity) which is measured by a power sensor (40) and is to be supplied to the impedance matching device (34).