Noise detection is performed by using the output of the phase comparator that the PLL comprises. The phase comparator outputs a signal that is based on the phase difference between the output of the voltage controlled oscillator and the reference signal. The phase difference reflects the effect of noise on the PLL and, in addition to the characteristics of the noise itself, such as the wave height value of the noise and the frequency component thereof, reflects the tolerance of the PLL to noise, whereby the level of risk that the system can actually be caused to malfunction can be judged.