A radio-wave half mirror for millimeter waveband is fixed inside a transmission line propagating electromagnetic waves of millimeter waveband in a single mode so as to transmit a part of incident electromagnetic waves and reflect a part thereof. The radio-wave half mirror includes: a half mirror body where a slit for transmitting electromagnetic waves is provided on a metal plate; and a dielectric plate that is provided on one surface side of the half mirror body so as to form a dielectric resonator which resonates at a frequency determined by the thickness and the permittivity, and has a transmittance characteristic having a degree of inclination substantially the same as that of the half mirror body in a slope which is inverse to a slope of a transmittance characteristic of the half mirror body in a desired frequency range of the millimeter waveband.