A clock generating apparatus includes a phase-difference measuring device for measuring a difference in phase between a reference clock and a feedback clock generated by a divider with a high-speed clock generated by a multiplier, an averager for averaging the measured phase difference, and an output clock generator for returning a self-generated output clock to the multiplier and the divider and generating an output clock synchronized with the reference clock by using the averaged phase difference and a generated operation clock. The multiplier generates the high-speed clock by multiplying the returned output clock, and the divider generates the feedback clock by dividing the returned output clock A frequency of generation of the output clock in the output clock generator is increased.