PURPOSE: To reduce generation of water leakage by fitting a non-expansion rubber layer having U-shaped cross section and a water expansion rubber layer having T-shaped cross section fitted in this U-shaped rubber layer into a long groove formed on a flanged portion of segments.

CONSTITUTION: A waterstop member 11 is formed by combining a U-shaped non-expansion rubber 12 with a T-shaped water expansion rubber 13 and this water stop member 11 is fitted into a long groove 22 of a flange 21 at the part of the non-expansion rubber 12 and bonded with an adhesive agent. The part of the waterstop member 11 is made of a water expansion rubber 13 on its outer peripheral surface and designed to come into contact with penetrating water and produce an expansion pressure, thereby stopping water. Then, the non-expansion rubber 12 is U-shaped so as to support the water expansion rubber 13 laterally, thereby constituting structure which inhibits lateral expansion. A more effective capacity to increase waterstop pressure is provided by producing an expansion pressure at a right angle to the plane of the flange 21 in order to make vertical expansion more effective. This construction makes it possible to reduce generation of water leakage dramatically.