PURPOSE: To ensure sealing by inserting a balloon-shaped inside supporting body into a sewerage main pipe and expanding it by means of air and the like to block a lower opening and then taking it out, in a construction method wherein a supporting pipe is inserted into the connecting hole of the sewerage main pipe and sealed by mortar to connect a drain pipe.

CONSTITUTION: A supporting pipe 4 is inserted into the connecting hole 2 of a sewerage main pipe 1 and also mortar 20 is injected therein. To connect the supporting pipe 4 to a drain pipe, a flange portion 6 to be made to abut against the sewerage main pipe 1 is provided around the outer periphery of the inserted portion 5 of the supporting pipe 4 and is used for blocking the space between the connecting hole 2 and the supporting pipe 4. A balloon-shaped inside supporting body 14 is inserted into the sewerage main pipe 1 and is expanded by fluids such as air, water and the like and thus the pace 25 is blocked by the inside supporting body 14, and mortar 20 is injected therein and solidified. Thereafter, the inside supporting body 14 is taken out. As a result, sealing of the connecting portion is ensured and sealing work is extremely simplified.