An oscillation circuit is constructed by providing a line and a Gunn diode on a dielectric substrate, and an oscillation signal as the fundamental frequency signal of an oscillator is injected into the Gunn diode through a DC bias line. By disposing a dielectric stripline between upper and lower conductor plates, at NRD guide as an output transmission line is constructed, and the line and the NRD guide are coupled. The cutoff frequency of the NRD guide is determined so that the fundamental wave component of oscillation signal of the oscillation circuit is cut off and a harmonic is propagated.