The movements of a person sitting on a seat cushion causes the unraveling and severing of sewn portions between a plurality of trim cover pieces. For this reason, two (central portion and left portion) trim cover pieces 22, 24 comprise skin materials 22a, 24a and wadding materials (first wadding materials 22b-1, 24b-1 and second wadding materials 22b-2, 24b-2). The first wadding materials are formed shorter than the skin materials and the second wadding materials, and the skin materials and the first wadding materials are bonded at a position separated a distance Y from the end of the first wadding materials, forming a bonded portion 30. An end of the skin materials is bent along an end of the first wadding materials, superimposed with the second wadding materials, and bent, the skin materials and the second wadding materials are then sewn together, thus forming a first sewn portion 31; then, at the first sewn portion, by bending back the skin materials and the second wadding material, a folded up portion is formed, and there, the skin materials and the second wadding materials are sewn, thus forming a second sewn portion 32.