PURPOSE: To prevent the digital circuit from being overheated due to the increase of the number of clock frequencies in the digital circuit consisting of CMOS elements.

CONSTITUTION: When the current amount (i) from a digital circuit part 1 is increased, a sensor part 2 raises the output voltage value v0 and a clock control part 3 lowers the operation clock frequency f0. When the current amount (i) from the digital circuit part 1 is decreased, the sensor part 2 lowers the output voltage value v0 and the clock control part 3 increases the operation clock frequency f0. Thus, even when the operation clock frequency f0 is increased and the digital circuit part 1 is going to overheat, since the current amount (i) from the digital circuit part 1 is increased in this case, the output voltage value v0 from the sensor part 2 is raised and the operation clock frequency f0 from the clock control part 3 is lowered. Thus, overheat of the digital circuit part 1 can be prevented and there is no malfunction of the digital circuit part 1 due to overheat and thermal breakdown.