To reduce the number of parts with a simple configuration by selecting a maximum gain setting signal to make a detected voltage signal equal to a maximum reference voltage signal and selecting a minimum gain setting signal to make the detected voltage signal equal to a minimum reference voltage signal, thereby outputting the signals to a gain control amplifier as a control voltage signal.

A gain control circuit 21 outputs the maximum (minimum) reference voltage signal DVrefmax (DVrefmin) optionally setting the maximum (minimum) transmission power of a transmission signal according to setting from outside. A first (second) voltage comparator 17 (18) inputs the detected voltage signal Vdet to its minus side and the maximum (minimum) reference voltage signal DVrefmax (DVrefmin) to its plus side. When a switch means 19 is connected to a side A (B), the control voltage signal Vcont is adjusted always to Vdet=DVrefmax (DVrefmin) to control the gain of the gain control amplifier 15 to be constant.