To provide an automatic matching device for a plasma processing device that effectively suppresses hunting without causing a decrease in matching speed.

A controller 90 includes: first and second matching adjustment sections 100 and 102 that perform variable control over electrostatic capacities of first and second variable capacitors 80 and 82 through first and second stepping motors 86 and 88 so as to bring an absolute value-measured value ZMm and a phase measured value Zθm of load-side impedance obtained from an impedance measurement section 84 close to a predetermined absolute value reference value ZMs and a phase reference value Zθs, respectively, as much as possible; first and second electrostatic capacity monitor sections 108 and 110; and a gain control section 112. The gain control section 112 performs variable control over at least one of proportional gains of the first and second matching adjustment sections 100 and 102, based on current values NC, NC2 of electrostatic capacities of the first and second variable capacitors 80, 82 which are respectively obtained from the first and second electrostatic capacity monitor sections 108 and 110.