To provide a sealing device which accurately folds the tongue of an envelope at a folding line across the entire width of the envelope.

Plural folding roller devices consisting of a folding roller 1, an auxiliary folding roller 2 installed coaxially on the end face below the folding roller 1, a drive gear 3, a rotating shaft 4 and a bearing 5 respectively, are linearly arranged at a specified interval. All of the folding rollers 1 are rotated in a direction shown by an arrow R1. Further, an auxiliary folding roller 10 whose rotating shaft is set orthogonally with the rotating shaft of the folding roller 1A, is installed on the lateral faces of a folding roller 1A and an auxiliary folding roller 2A on the extreme upstream side. In addition, a folding guide plate 11 and a pressure contact roller 12 which comes into contact with the lateral face of an auxiliary roller 2B on the extreme downstream side are provided on the downstream side of the auxiliary folding roller 10.