A controller 90 of an automatic matching unit includes a first and a second matching control unit 100, 102 for respectively variably controlling the electrostatic capacitances of a first and a second variable capacitors 80, 82 through a first and a second stepping motor 86, 88 such that a measured absolute value ZMm and a measured phase Zθm of a load impedance obtained by an impedance measuring unit 84 become close to a predetermined reference absolute value ZMs and a predetermined reference phase Zθs, respectively; and a gain control unit 112. The gain control unit 112 variably controls a proportional gain of at least one of the first and the second matching unit based on current electrostatic capacitances NC1 and NC2 of the first and the second variable capacitors 80, 82 obtained by a first and a second electrostatic capacitance monitoring unit 108, 110, respectively.