A branch pipe lining method is provided which can directly achieve an air-tight connection between a pressure bag and a branch pipe liner bag in a simple structure without employing a conventional sealed tube. An open end of the pressure bag is attached to a set nozzle attached to a work robot which is movably installed in a main pipe. A branch pipe liner bag, impregnated with a thermosetting resin, is placed inside the pressure bag. The liner bag has a flange which is removably attached to the set nozzle by fitting a protrusion formed on a surface of the flange into a concave groove formed in the set nozzle. With the flange being closely contacted to the inner wall of the main pipe around a branch pipe opening, compressed air is supplied into the pressure bag to evert and insert the branch pipe liner bag into a branch pipe. While the branch pipe liner bag is pressed against the inner wall of the branch pipe, the thermosetting resin impregnated in the branch pipe liner bag is cured, whereby the inner wall of the branch pipe is lined with the cured thermosetting resin, i.e., the branch pipe is repaired. Upon completing the lining operation, the protrusion is disengaged from the concave groove to remove the work robot and the pressure bag from the branch pipe liner bag.