PURPOSE: To produce a clock which actuates a microcomputer at a speed approximate to the highest processing speed by providing a circuit which detects the power supply voltage and produces a clock signal slightly lower than the maximum working clock frequency that is decided in response to said detected power supply voltage.

CONSTITUTION: The value of voltage applied to an input terminal for power supply voltage is converted into a digital signal 14 by an A/D converter 21. An oscillator 22 delivers a clock 12 of a prescribed frequency, and a divider 23 devides the clock 12 and delivers a clock signal 13 having plural different frequencies. A data selector 24 selects one of the signals 13 in response to the signal 14. This selected signal 13 is applied to a microcomputer 25 in the form of a clock signal 15. In such a way, it is possible to obtain a generating circuit for microcomputer clock which actuates the computer 25 at a speed approximate to the highest processing speed corresponding to the power supply voltage.