Abstract: A clock signal generator comprising a first oscillator which normally supplies clock pulses to the output of the clock signal generator, a first counter for counting the pulses generated from the first oscillator and producing a carry signal after counting n1 pulses generated from the first oscillator, a second oscillator for producing clock pulses for possible back-up purpose, a second counter for counting the pulses generated from the second oscillator and adapted to produce a carry signal after counting n2 pulses generated from the second oscillator and to be reset by the carry signal from the first counter, n2 being larger than n1, and a control circuit which blocks the output from the second oscillator as long as no carry signal is supplied thereto from the second oscillator.