A socket body and a header body are respectively reinforced by inserted reinforcing members, so that a protruding table in a plug groove of the socket body is eliminated and a width dimension thereof is reduced. The socket body is formed in a flat rectangular parallelepiped shape by resin molding, and a pair of socket reinforcing members is inserted into both end portions in longitudinal direction, each having a pair of fixed portions respectively protruding outward from lower ends of side walls of the socket body and a coupling portion for coupling between a pair of the fixed portions. A plug groove of substantially rectangular shape is formed at center portion of the socket body, and plural pairs of socket contacts are respectively arranged in two lines on both side walls in longitudinal direction along the plug groove. Header reinforcing metal fittings formed on the same base metal and having substantially the same cross section as that of the header posts are integrally inserted into both end portions of the header body in the longitudinal direction.