PURPOSE: To present an oscillator where the variance of frequency is small even if external conditions are changed, by inserting a resistance in series to a coupling loop in a coupling circuit between the local oscillator and the next- stage circuit and matching the impedance of the next-stage circuit and that of the coupling circuit to each other.

CONSTITUTION: A coupling loop 13 includes a terminating resistance 16 to form a directional coupler, and the impedance of the coupling circuit viewed from a buffer transistor TR15 is matched to the input impedance of a buffer TR15 by a matching capacitor 17 and a coupling capacitor 14, and thus, wide-band and stable coupling between a local oscillator 4 and a buffer amplifier 6 is realized. That is, the influence of said variance is not transmitted to the local oscillator 4 because of the action of said directional coupler even if the constant of the buffer amplifier 6 is changed, and consequently, the frequency of the local oscillator 4 is kept stable.