A method for manufacturing including securing attachments disposed near both ends in the seat core member front portion made of a molded article of expanded beads, the securing attachments being connected by an embedded connecting member extending in the seat core member longitudinal direction. The securing attachment includes a base embedded in the molded article of expanded beads, an upright frame joined to the base to extend out of the molded article of expanded beads, and a securing portion extending from the upright frame to project away from the molded article of expanded beads. The method includes creating, during or after molding the molded article of expanded beads, a space or notch in an outer side of the securing attachment upright frame of the molded article of expanded beads in the seat core member longitudinal direction, the notch extending toward an outer side in the seat core member longitudinal direction.