PURPOSE: To prevent the timewise software processing from being changed by correcting fluctuation of an oscillating frequency of a CPU rationally regardless of using a low cost CR oscillator.

CONSTITUTION: When a power supply voltage and an ambient temperature are set under a reference measurement condition, the measured oscillating frequency of a CR oscillator is stored in an E2PROM 16 as a reference oscillating frequency. On the other hand, a current voltage is detected by a voltage detector 14 and a current temperature is detected by a temperature detector 15. The frequency fluctuation from the oscillating frequency under a reference measurement condition is corrected by an arithmetic operation predetermined by a CPU 11 based on the oscillating frequency stored in the E2PROM 16 and the current voltage and temperature and an accurate frequency at present is obtained indirectly.