PURPOSE: To form a branch pipe easily on the side of a pipe, by putting a forming member consisting of a tapered part and a columnar part in a pipe having a hole, and forming a branch tube by pulling out the forming member and fitting to the pipe.

CONSTITUTION: A pipe A to be worked having a proper hole at specified position is set on a base stand 3, and fixed with a holding member having a hole 2. A columnar member 12 corresponding to inner diameter of a branch pipe B and a forming member 14 having a tapered part in the upper part 13 are inserted in the pipe A. The hole of the pipe A is passed through the pulling out end 15 in the upper part of the forming member 14, and a connecting ring 16 of a pulling device is connected. By pulling out the forming member 14 by a hydraulic device 7 through the hole, open rim of the hole is bent outward along the tapered part 13, and a branch pipe B having necessary diameter is formed by the columnar part 12.