A method of forming a molded seal portion onto a corner part of a weather strip without generating any molding defect due to variations in the injection amount, injection pressure or the like of the molding material. A seal portion projecting from one side wall of a trim portion is partly cut off, the remaining trim portion is bent to a corner configuration and placed in a mold, and a molding material for the seal portion is injected into a cavity of the mold from the position corresponding to a longitudinally extending base end edge of the molded seal portion such that the molding material flows from the side of the longitudinally extending base end edge of the molded seal portion towards a distal end edge thereof to fill the cavity.