An oscillator on a Gunn diode with an open resonator is described. The diode is located in a cut of one of two mirrors, which consists of two parts. This mirror simultaneously plays a role of the electrodes and cooling system. The main characteristics of the oscillator operating at a frequency of 40 GHz are as follows: 30 mW (the output power), 20 kHz (the oscillation linewidth), 10-7 (the relative short-term stability), and up to 2 GHz (the range of mechanical tuning within a single zone at a slope of the tuning characteristic of 1.19 GHz/mm).