Abstract: A clock supply apparatus contains: a clock generating unit for generating a clock signal; a phase comparing unit for inputting a reference clock signal, and obtaining a phase difference between the reference clock signal and the clock signal generated by the clock generating unit; a frequency control unit for generating a control signal for controlling the frequency of the clock signal generated by the clock generating unit, based on the phase difference; a clock preciseness monitoring unit for monitoring preciseness of the clock signal, based on the phase difference; and a clock preciseness output unit for outputting information indicating the preciseness of the clock signal, based on the phase difference.