PURPOSE: To automatically measure continuously at all times, and obtain accurate and correct measuring values.

CONSTITUTION: A laser slit light projector 3 and a CCD camera 4 are set on an inner face of a skin plate 1 of a shielding machine. The laser slit light projector 3 projects a laser slit light S towards the end face of a segment 2 assembled in the back, and the CCD camera 4 photographs the vicinity of the end face of the segment 2 illuminated by the laser slit light S. When the laser slit light S is projected from the laser slit, light projector 3 towards the end face of the segment 2 and the vicinity of the end face of the segment illuminated by the laser slit light S is photographed by the CCD camera 4, a computer 5 detects by image processing the position of the end face of the segment illuminated by the laser slit light S which is displayed on a screen. A difference of the detected value and a value of the position of the skin plate calculated beforehand in accordance with the jack stroke is calculated as a value of the tail clearance.