A predetermined component such as a recliner is attached to a long member in advance, and thereafter by bending the long member, a pair of side frames and an upper frame are integrally formed. Therefore, the recliner can be attached to the long member not bent yet and thus in a substantially flat state, and in addition, the pair of side frames and the upper frame can be integrally formed, which does not require labor and contributes to a reduction in manufacturing cost. Moreover, by adjusting a position where the bending is performed, it is possible to manufacture seat frame structures different in dimension even if the long members not bent yet have the same length, and from this viewpoint as well, it is possible to reduce labor and manufacturing cost.