To solve the problem of characteristics variance or low efficiency in the work of assembly although a pill type package integrated with a messa type gun diode element is located inside a waveguide in the conventional gun diode oscillator utilizing a secondary higher harmonic mode.

A microstrip line 14 for oscillation part is formed on the upper surface of a dielectric substrate 31 of a prescribed width having a ground conductor 23 on the rear face thereof, a slot line 19 coupled with the microstrip line 14 in the manner of circuit in one terminal part is formed on the ground conductor, a surface-mounted gun diode 13 is packaged on the microstrip line 14 so that a substrate assembly can be provided. Then, this substrate assembly is inserted into a rectangular waveguide to comprise a fin line type output part, and the cutoff frequency of this fin line is set higher than the oscillation frequency of a basic wave.