To prevent the occurrence of wrinkles when the extended side is folded back from a side edge along a conveyance direction of a conveyor belt in a sheet.SOLUTION: A folding device 1 includes a conveyor belt 21, a suction device 22, and a folding guide 3 for folding back a side 12s of a continuum 12r of an exterior material in the width direction CD. The folding device folds back both sides 12s, 12s of the continuum 12r while conveying them in a state where the side 12s of the continuum 12r of the exterior material is extended to the outside from a side edge 21s of the conveyor belt 21. The suction device 22 is disposed in a sheet conveying area 20. The folding guide 3 has a side wall 31 disposed at both side edges 21s, 21s of the conveyor belt 21, and an extended part 32 extending inside from the side wall 31. An inner angle θ2 formed by the side wall 31 and the extended part 32 is configured such that the angle of the upstream side is larger than that of the downstream side.SELECTED DRAWING: Figure 3【課題】シートにおける搬送ベルトの搬送方向に沿う側縁から延出した側部を折り返すときに、皺の発生を抑制させること。【解決手段】搬送ベルト２１と、吸引装置２２と、外装材の連続体１２ｒの側部１２ｓを幅方向ＣＤに折り返す折り返しガイド３とを備え、外装材の連続体１２ｒの側部１２ｓが搬送ベルト２１の側縁２１ｓから外方に延在した状態で搬送しながら該連続体１２ｒの両側部１２ｓ，１２ｓを折り返す折り返し装置１である。吸引装置２２はシート搬送領域２０に配されている。折り返しガイド３は、搬送ベルト２１の両側縁２１ｓ，２１ｓに配される側壁部３１と、側壁部３１から内方に延出する延出部３２とを有している。側壁部３１と該延出部３２とのなす内角θ２は、下流側よりも上流側の方が大きい。【選択図】図３
