To present a clock generator capable of spreading the spectrum of oscillation frequency by simple control in a small additional circuit, and its control method. A phase locked loop circuit is provided from a frequency phase comparator 11, an output clock signal PO is outputted from a voltage control oscillator (VCO) 14 by way of a charge pump circuit (CP) 12 and a loop filter (LF) 13, and is returned to the frequency phase comparator 11 by way of a frequency divider (DIV) 15. Detecting the phase difference of reference clock signal R and divided clock signal D, and locking the oscillation frequency of the output clock signal PO to specified frequency, a modulation signal M is outputted from a modulation pulse generator 1 regardless of phase locked control of phase locked loop circuit, and is superposed on phase comparison signal P, and thereby the oscillation frequency of output clock signal PO is modulated. An output clock signal PO having a predetermined spectrum spread characteristic can be obtained.