A weather strip has a mounting base portion which is mounted on a flange and a seal portion which is integrally provided on the mounting base portion, so as to seal a gap between a vehicle body opening open/close member and a vehicle body opening peripheral part. The mounting base portion is provided with at least an exterior side wall and a bottom wall, and a connecting portion between the exterior side wall and the bottom wall is formed more rigid than the other portions of the mounting base portion, a pressure sensitive adhesive double sided tape being bonded to an interior side surface of the exterior side wall. A flange contact portion which holds a distal end of the flange is formed on an inner surface of the bottom wall at a position in the vicinity of the connecting portion with the exterior side wall, while a caulking member which seals the distal end of the flange is mounted on the inner surface of the bottom wall except for the portion where the flange contact portion is formed.