A PLL circuit includes a PLLic formed into an integrated circuit; a loop filter for receiving an output signal from the PLLic; a voltage-controlled oscillator having an oscillation frequency which is controlled according to an output signal of the loop filter for applying a controlled oscillation output signal to the PLLic, the voltage-controlled oscillator including a resonator and a negative resistor circuit; wherein a buffer amplifier functioning as a part of the voltage-controlled oscillator is incorporated into the PLLic, and the resonator and the negative resistor circuit of the voltage-controlled oscillator are disposed outside of the PLLic.