To improve productivity by applying the constitution that a stuffing material made of a body formed out of hard cotton and soft cotton covered with top fabric is laid on the upper surface of a spring body structure, and both ends of a member having the prescribed length are caulked after passed through the suitable position of the stuffing material.

A stuffing material is attached to a mirror-finished surface as the upper surface (lower surface) of a spring body structure. The stuffing material has a three-layer structure with a body of hard cotton 12a and soft cotton 12b covered with top fabric 14. Also, a cutter 170 cuts a caulked member 160 and this caulked member 160 so cut is displaced and held as a chip 161 in a cylindrical part 91. A pair of front end forming rods 171 clamp the chip 161, and form the forward end of the chip 161 to a taper. Then, the chip 161 is pressed down, and passes the top fabric 14 of the stuffing material, the soft cotton 12b and the hard cotton 12a. Finally, the chip 161 arrives at the recess 97 of the lower die 96 of a jig 90. Then, the cylindrical part 91 and a pressing cap 93 are integrally lowered, and caulk the upper and lower ends of the chip 161, thereby forming a holder part.