PURPOSE: To enable the sealing without using a special material by forming a seal of a double wall elastic sleeve having single or plural continuous cavities, and inserting the seal at the end of a pipe so as to be accurately positioned in a surface of a round window passing through a riser of a manhole.

CONSTITUTION: A riser 11 of a manhole is formed with a round window for pipe 12. An uninflated seal 13, namely, the seal 13 in the condition that it is not expanded yet is arranged in the pipe 12 at an accurate position in a curved surface 15 so as to be relatively positioned at a central part of the round window 14. When the seal 13 is positioned once, the pipe 12 and the seal 3 are moved as a unit into the round window 14.