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).