PURPOSE: To make it possible to adjust a frequency without an adjustment by coupling a resonator, coupled with a modulator, with the transmission line of a resistive terminator connected to the gate of FET whose drain is grounded.

CONSTITUTION: To gate 2 of FET1 with drain 4 grounded, transmission line 5 to which resistive terminator 11 has been coupled is connected. To this transmission line 5, resonator 8 where modulating circuit 9 with varactor 10 has been coupled is connected. Then, a modulator output is led out from source 3 via transmission line 12. Then, coupling resonator 8 at a position λg (wavelength of resonance frequency) away from the gate side satisfied the oscillation conditions, causing oscillation at a resonance frequency determined by resonator 8 and modulating circuit 9. Therefore, frequency modulation can be brought to effect without any adjustment by applying a modulated signal to diode 10.