A digitally-controlled oscillator circuit receives a digital value and generates a driving signal for driving an oscillator at a frequency according to the received digital value. A time-to-digital converter circuit receives a detection signal of oscillation of the oscillator, receives the driving signal, and detects a phase difference between the detection signal and the driving signal. A control circuit receives the detected phase difference and controls the frequency of the driving signal generated by the digitally-controlled oscillator circuit, such that the detected phase difference coincides with a predetermined resonant phase difference to resonate the oscillator.