PURPOSE: To form a mold at the zone between end faces of two sewage pipes by inserting end portions of a smooth-surfaced cylindrical elastic leakproof bag into the end portions of the two sewage pipes opening into a space and by feeding pressurized water into the bag to expand it.

CONSTITUTION: The ends of an elastic leakproof bag 1 are inserted into the end portions 5 and 6 of sewage pipes 3 and 4 opening into a space 2, and pressurized fluid is fed through an injection port 7 into the bag 1 to expand it. Thus, the bag 1 forms a cylindrical body between the end portions 5 and 6 of the pipes 3 and 4. Concrete is cast in the bottom of the space 2 as far as the lower half portion of the cylindrical body is burried in the cast concrete, to form a groove which communicates the lower half portions of the pipes 3 and 4.