To prevent secular deformation of a slit formed on a mounting base part by impact pressure of a vehicle panel and a door panel in a method for manufacturing a weatherstrip with a corner by forming an extrusion-molded weatherstrip and a corner part by an injection molding mold and uniting and integrating both of the weatherstrip and the corner part.

At almost the center parts of long sides of the slits 11 and 12, a projection piece 8 is provided on the one long side 9 and is brought into contact with the opposed long side 10 in a bridge shape.