PURPOSE: To obtain an electronic clock that ticks time irrespectively of whether a commercial electric power source is supplied or interrupted by selecting and counting the outputs of the commercial power source or an oscillating circuit according to the supplying condition of the commercial power source.

CONSTITUTION: When a commercial power source is supplied, a power supply detecting circuit 5 detects the same and supplies the signal formed by waveform shaping 4 of the commercial power source to a counter 7. When the commercial power source is interrupted, a selecting circuit 6 is changed over by the output of a power supply detecting circuit 5 to the side of a frequency dividing circuit 2, which supplies the signal divided down 2 from the output of an oscillating circuit 1 to the counter 7.