A variable frequency amplifier includes a main amplifier system 4 for amplifying one of signals into which an input signal is split by a directional coupler 3 to output the amplified signal, and an injection amplifier system 9 for adjusting at least one of the amplitude and phase of the other one of the signals into which the input signal is split by the directional coupler 3 according to a setting provided thereto from outside the variable frequency amplifier, and for amplifying the other signal and injecting this amplified signal into an output side of the main amplifier system 4.