PURPOSE: To prevent a seal member from being projected out of a gap even if is pushed by both one and the other members of a joint section in the seal member used for the joint section of bridge beams.

CONSTITUTION: The seal member S provided in a gap formed by the telescopic motion of each bridge beam between one member (the extended section 6a of a first surface metal plate 6) and the other member (a second surface metal plate 9), is made up of a cylindrical body 11 which is located in the gap, and has oil filled in, a first flange 12 which is provided for the upper side of the cylindrical body 11, and is fixed onto the extended section 6a, and of a second flange 13 which is provided for the upper side of the cylindrical body 11, and is fixed onto the second surface metal plate 9. When the extended section 6a and the second surface metal plate 9 come close to each other, the upper side of the cylindrical body 11 is pressed from both sides, the lower side of the cylindrical, body 11 is expanded by oil, as a result, the seal member S is not projected upward out of the gap.