The present invention may make it possible to efficiently form reinforcing deformed portions in a core buried in the trim, and may make it possible to sufficiently secure the rigidity of the core, the rigidity based on reinforcing deformed portions. The method of manufacturing an elongated core which is buried in an elongated trim. A longitudinal direction of the elongated core is a first direction and a width direction of the elongated core is a second direction, the method comprises: the slit forming being performed by supplying an elongated strip-shaped metal plate to between a pair of slitter rollers engaged with each other, forming slits, which extend in the second direction in a state in which the slits are not formed to a portion of the strip-shaped metal plate in the second direction, at predetermined intervals in the first direction so as to form the core pieces between the adjacent slits; and the rolling being performed by supplying the strip-shaped metal plate having the slits formed therein to between a pair of rolling rollers, rolling the portion of the strip-shaped metal plate in which the slits are not formed in the second direction to form a rolled portion extending in the first direction, and expanding the slits in the first direction to form the spaces, and supplying the primary core to between a pair of deformed portion forming rollers engaged with each other such that reinforcing deformed portions extending in the second direction are repetitively formed in the first direction, wherein the convex and concave deformed portions are alternately formed in the first direction, and wherein the convex and concave deformed portions are formed so as to be continuously S-shaped in a cross section along the first direction.