A variable frequency resonator includes a series resonance circuit and a parallel resonance circuit. The series resonance circuit includes a first inductor, a first capacitor and a variable capacitance diode which are connected together in series. A reverse bias voltage is applied to the variable capacitance diode. A signal source is connected to the series resonance circuit. The parallel resonance circuit includes a second inductor and a second capacitor which are connected together in parallel. One terminal of the parallel resonance circuit is connected to a connection point between the first inductor and the first capacitor, and the other terminal of the parallel resonance circuit is connected to a connection point between the variable capacitor and the one terminal of the signal source.