To provide a core embedded in a weather strip which can prevent a disadvantage caused by a core piece which is torsionally deformed when roll-forming the core in a U shape in its cross section.

By forming a plurality of spaces 24 at a predetermined interval in the longitudinal direction of a core raw material by punching, a core 16 in which core pieces 22 and spaces 24 are alternately provided in the longitudinal direction is formed. Prior to roll-forming the core 16 into a U shape in its cross-section, the core 16 is applied with pre-twist to torsionally and plastically deform the core pieces 22 in a particular direction (a direction opposite to a direction where the core pieces 22 are torsionally deformed by the roll-forming) in advance. Then, by torsional deformation of the core piece pieces 22 when the core 16 is roll formed into the U shape in cross section, the torsional and plastic deformation of the core pieces 22 by the pre-twist is corrected so as to make ends of the core piece parts 22 in the width direction align in a straight line in the longitudinal direction and to avoid generating differences between edges of the core pieces 22 adjacent to each other in the longitudinal direction.