An impedance adjustment device includes a variable capacitor unit. A microcomputer changes the capacitance value of the variable capacitor unit by switching PIN diodes included in n capacitor circuits on or off separately. Thus, the impedance on the plasma generator side when viewed from a high frequency power supply is adjusted. When changing the capacitance value of the variable capacitor unit to a target capacitance value, the microcomputer changes the capacitance value. When a predetermined period passes after the change of the capacitance value, the microcomputer changes the capacitance value again.