A PLL circuit which outputs an oscillation clock signal synchronous with a reference clock includes a phase lock detector for detecting if the oscillation clock signal is synchronous with the reference clock. If the phase lock detector detects a phase difference between the oscillation clock signal and the reference clock, a charge pump circuit is used to alter the oscillation clock signal so that the oscillation signal is placed back in sync with the reference clock. The charge pump selects one of a ground potential and a power supply potential in response to a comparison result of the oscillation clock signal and the reference clock. The charge pump pulls a constant current to ground from an output terminal of the charge pump circuit when the ground potential is selected and supplies a constant current to the output terminal of the charge pump circuit when the power supply potential is selected, thereby producing an output which alternately repeats the ground potential and the power supply potential.