A PLL circuit includes a phase comparator for outputting a frequency control signal based on a result of comparison between phases of an input reference clock signal and a fed-back oscillation signal; an oscillation circuit, connected to the phase comparator, for outputting an oscillation signal having a frequency in accordance with the frequency control signal, a power source voltage, and a predetermined reference voltage; and a bias control circuit, connected to the oscillation circuit, for increasing either the potential difference between the reference voltage of the oscillation circuit and a ground voltage or the potential difference between the power source voltage of the oscillation circuit and the ground voltage. A transistor in the oscillation circuit can operate in a saturation area, thereby operating the PLL circuit at a high speed with a low power source voltage, without being easily affected by a variation in the temperature or other process conditions.