In a jitter generator (phase modulator), worsening of phase noise is restrained and phase modulation accuracy is improved, and the phase modulation accuracy is improved by preventing a change in the detection sensitivity of the phase detector, if any, from affecting a change in the phase modulation index. Also, phase modulation is made possible without lowering the phase modulation accuracy even when an input phase signal increases. In a jitter generator using a PLL circuit, a quadrature modulator to which a modulation signal from a phase signal generator is provided is inserted to an input stage of a phase detector that constitutes the PLL circuit. In addition to this, an overflow detector that detects an overflow on an upper limit side or lower limit side of an analog/digital converter, a control unit that outputs a value for an effective region of the analog/digital converter on the basis of an output of the overflow detector, a digital/analog converter that coverts an output of the control unit to an analog signal, and an adder that adds an output of the digital/analog converter to the modulation signal, are provided.