PURPOSE: To convert easily with a high accuracy a millimeter to an inch by converting a moving extent of a moving body to a distortion quantity, converting its distortion quantity to an analog signal by a strain gauge, and also converting it to a digital signal.

CONSTITUTION: A spindle 2 being a moving body is moved relatively to a frame body 1. A spring means 4 contacts to a spring hooking part 3, and converts a moving extent of the spring hooking part 3, namely, a moving extent of the spindle 2, to a distortion quantity. A distortion quantity converting part is formed by the spring means 4 and the spring hooking part 3. A strain gauge 10 is fixed to the spring means 4 and the distortion quantity is converted to an analog signal A. Subsequently, it is converted to a digital signal B by an A/D converting part 11, and visualized and displayed by a display part 12.