PURPOSE: To decrease the fluctuation of a microwave oscillation frequency due to thermal fluctuation of an oscillation element caused at application of pulse by adopting the constitution such that a pre-bias below a microwave oscillation voltage is impressed just before application of pulse.

CONSTITUTION: A pre-bias (b) with a voltage lower than a voltage reaching the oscillation is impressed to a position just before a pulse (a) biasing a gun diode up to its oscillation region in the microwave oscillating method oscillating the microwave through the application of a pulse to a solid-state microwave oscillating element. Thus, the gun diode itself is sufficiently heated by the application of the pre-bias (b) to cause thermal balancing state. Since the pulse (a) is applied in the succeeding timing, the microwave with a stable frequency is oscillated without being affected by heat in this case.