There is disclosed herein a millimeter-wave/submillimeter-wave oscillator comprising a mirror and a diffraction grating so as to form a Fabry-Perot type resonator. The mirror, being a concave mirror or a plane mirror, and the diffraction grating are disposed in facing and spaced relation to each other. The diffraction grating has a plurality of channels extending in a direction normal to a surface of the mirror so that the oscillator has a comb-like configuration. A plurality of oscillation elements are mounted on portions within the channels of the diffraction grating.