PURPOSE: To obtain the impedance of an active element efficiently and simply by fitting a jig, which has a connector and a coaxial cable, to the fitting part of the active element of a high-frequency active circuit as an impedance measuring part and by measuring the reflection coefficient of a measured I/O circuit from the connector of this jig.

CONSTITUTION: Impedance measuring jig 9 which has connector 11 functioning as an impedance measuring part in stead of an active element and coaxial cable 10 with one end connected to connector 11 is fitted to the fitting part of the active element of a high-frequency active circuit. One end of coaxial cable 10 in jig 9 is connected to the part where the active element of output matching circuit 3 is fitted conventionally, and connector 11 is connected to network analyzer 8 for reflection coefficient measure, and terminal resistance 6 is connected to connector 5 for signal output, thereby performing measurement. For impedance measurement of the input matching circuit, the direction of impedance measuring jig 9 is opposite, and terminal resistance 6 is connected to the connector for signal input, thereby performing measurement. Thus, the dynamic impedance of the active element is obtained.