PURPOSE: To remove an effect due to mutual coupling of two piezoelectric oscillators having different frequencies by dividing respective outputs of two oscillating circuits using said oscillators and fetching the frequency difference from the divided frequencies.

CONSTITUTION: The frequencies of oscillators (c) and (d), for instance, assume 32768Hz and 43690.666Hz, respectively. A one third divider is inserted to a frequency divider 18, and a one fourth divider to a frequency divider 19, a detection of the frequency difference is detected 20 and it is sent to a storage circuit 14, a pulse interruption-erasing circuit 15, a frequency divider 16 and an indicator 17.