An output monitor circuit of a radio-frequency circuit outputs a radio-frequency signal of wavelength λ. The circuit includes a directional coupler, a detection circuit, and a back flow blocking circuit. The directional coupler has a coupling line of a length less than λ/4 coupled with an output line of the radio-frequency circuit and a terminating resistor connected to one end on the radio-frequency circuit side of the coupling line. A radio-frequency signal is derived from the other end of the coupling line by means of coupling between the coupling line and the output line. The detection circuit detects a radio-frequency signal derived from the directional coupler to produce a monitor signal. The back flow blocking circuit blocks signal components back flowing from the output side toward the output line in a portion where there is no coupling between the output line and the coupling line.