The present disclosure, calculating an impedance change considering a reflected wave due to IMD, provides an impedance matching device for performing impedance matching between a source power supply side and a load side, including: a detector that detects a forward wave power supplied from the source power supply and a reflected wave power from the load and outputs a forward wave voltage as a component of the forward wave power and a reflected wave voltage as a component of the reflected wave power, an impedance information output part that calculates impedance from the forward wave voltage and the reflected wave voltage, and a matching part that performs matching operation based on an impedance value supplied from the impedance information output part. The impedance information output part complexifies each of the forward wave voltage and the reflected wave voltage to calculate an impedance value to generate an impedance locus.