Abstract: An economical branch pipe liner bag is provided. The branch pipe liner bag allows for a reduction in manufacturing cost, stabilized quality, and mass-production, and does not have any portion to be wasted when used. One end of a tubular resin absorbent material impregnated with an unhardened hardenable resin is outwardly folded and pressed to form an extension which is sandwiched between separate flange forming members to form a flange at the one end of the tubular resin absorbent material, thus constructing the branch pipe liner bag. Since the branch pipe liner bag has the flange formed at one end by integrally coupling the flange forming members, which are members separate from the tubular resin absorbent material, the flange forming members can be mechanically mass-produced independently of the tubular resin absorbent material, with the result that a manufacturing cost of these flange forming members is reduced, and consequently a manufacturing cost of the branch pipe liner bag is reduced.