PURPOSE: To detect power passed through an output transmission line without using any directional coupler even when the standing wave ratio of the same transmission line is high.

CONSTITUTION: A 90° phase shifter 7 is connected to the output of a power amplifier circuit 1, a transmission output is obtained from the output of the phase shifter 7, a first detector 5a detects a high frequency voltage at the input terminal of the phase shifter 7, a second detector 5b detects a high frequency voltage at the output terminal of the phase shifter 7, the output voltage of the second detector 5b is added to the output voltage of the first detector 5b by an adder 8, and the output voltage of the adder 8 is defined as a passed power reference voltage. Thus, even when the standing wave ratio of an antenna is high, the passed power can be detected. Further, by calculating the square sum of the said high frequency voltages, detection error can be reduced.