[0029] Step S6: heating the first felt layer 11, the second felt layer 12, the waterproof layer 13 and the third felt layer 14 which have been joined together so as to melt the low-melting-point fibers of the second felt layer 12 and the low-melting-point fibers of the first felt layer 11 and not to melt the stuffing fibers of the first felt layer 11 and the stuffing fibers of the second felt layer 12.