A push-push oscillator including a resonator having a transmission line and a capacitance connected to the transmission line in parallel; an oscillating circuit responsive to the resonator for oscillating and for producing first and second outputs having an antiphase relation therebetween; an in-phase combining circuit for summing the first and second outputs of said oscillating circuit to produce a summed signal; and an antiphase combining circuit responsive to two components from the resonator having an antiphase relation for producing a differential signal in accordance with a difference between the two components. Alternatively, the in-phase combining circuit is connected to the resonator and the antiphase combining circuit is connected to the oscillating circuit. The antiphase combining circuit outputs a fundamental wave component of the resonator. The in-phase combining circuit outputs a second harmonic wave component. When this push-push oscillator is used in a PLL circuit to form a frequency synthesizer, a high frequency output is obtained with a low power consumption.