PURPOSE: To reduce fatigue given to an operator, and accurately and quickly patch up an inner peripheral edge by adjusting a tensioning screw while observing a position of the edge tip on a display, and putting the edge tip in an allowable position range preset on the diaplay.

CONSTITUTION: An electronic line is adjusted to a graduation (O) position, and the edge tip 9 is adjusted to the electronic line by using visual field positioning stands X and Y. An electronic line corresponding to a (O) position and an electronic line to display a desired allowable position range are displayed on an image screen of a display as two parallel lines. A position of the edge tip 9 of an inner peripheral edge simultaneously photographed by a CCD camera 10 is also projected on the image screen of the diaplay. A tensioning screw is rotationally lowered, and when tensile force is applied to the inner peripheral edge, the edge tip 9 in the vicinity of the rotationally lowered tensioning screw moves, and its condition is directly observed on the image screen of the display.