PURPOSE: To automatically control and adjust the position of a glass scale to an optimum position without any human intervention even if the glass scale has sticking dust or flaws to make a position signal abnormal.

CONSTITUTION: This device is equipped with a clock abnormality detecting circuit 10 which inputs a reference clock signal outputted by a PLL circuit 9 and detects whether or not the reference clock signal is abnormal, a glass scale position control circuit 11 which receives the result of clock signal abnormality decision from the clock abnormality detecting circuit 10 and generates and outputs a control signal for moving the position of the glass scale 5, and motor-driven micrometers 6 and 7 which receives the control signal and control and adjust the position of the glass scale 5 so that the abnormal clock signal becomes normal.