Abstract: In each of oil-retaining sheets, an end edge and side edges of a recess are separated from each other. The side edges are formed into tongue portions covered by first anti-wear meshes which prevent loosing of the side edges, and these tongue portions are maintained in sliding contact with their corresponding side walls of a guide portion of a guide rail. The end edge is covered by a second anti-wear mesh which prevents loosing of the end edge, and the second anti-wear mesh is maintained in sliding contact with an end wall of the guide portion. A gap is also formed to maintain the tongue portions and the second anti-wear mesh out of contact with each other.