PURPOSE: To prevent the breakdown of a semiconductor element to be measured by keeping the initial setting of the DC bias to be supplied to said element low, determining the max. point of the output electric power of that time while regulating an output side impedance regulating circuit, increasing the DC bias in that state and measuring the output power.

CONSTITUTION: A directional coupler 22, an output impedance regulating circuit 17 and an output wattmeter 18 are connected to the output terminal of a transistor 16 to be measured. The reflection power detected from the terminal for detecting reflection power disposed on the output side of said coupler 22 is supplied to a DC power source control circuit 21. This circuit 21 detects the signal level from the coupler 22, and controls an electric power source 19 to decrease the DC bias to be applied to the output terminal of the transistor to be measured. The initial set value of the DC bias is kept low, and the max. point of the output power of that time is determined while regulating the circuit 17. In this state, the DC bias is raised to the next stage, whereby the output power is measured.