A digitally controllable oscillator is provided with a variable-frequency ring oscillator comprising an odd number of inverting circuits connected to each other in a ring. The frequency of the output signal of the ring oscillator is determined by a digital input signal specifying the frequency of the output signal of the ring oscillator. The number of times of circulation of a pulse signal through the ring oscillator is counted. A pulse generator generates a pulse signal upon the coincidence of the counted number of times of circulation of the pulse signal through the ring oscillator with a number corresponding to the digital input signal. A series of these operations is repeated to make the pulse generator generate pulse signals successively at a period corresponding to the digital input signal.