PURPOSE: To improve the economical efficiency by installing a plurality of spacers having each proper height, permitting free replacement, onto a cutter body for the inside surface cutting of the branched pipe in a conduit line repairing method.

CONSTITUTION: Though a cutter 10 is used for cutting a part for closing the branched pipe 7 of a lining member 2, the length of the cutter 10 is not sufficient for the part to be cut in the work in a main pipe having a large diameter. In such a case, a spacer 12 is demounted in the reverse order to the installation of the cutter 10, and a spacer 12' having a large height is installed into a cutter body 11, and the height of the cutter 10 is increased, and the cutting work in the main pipe having the large inside diameter is permitted. Therefore, when a plurality of spacers 12, 12',... having diifferent height are prepared, the height of the cutter 10 can be varied arbitrarily only by installing the spacers 12, 12',... onto the cutter body 11 even if the inside diameter of the main pipe varies, and the need of preparing a plurality of expensive cutter bodies 11 is avoided and economical operation is permitted.