A sealing system for creating a sealed letter by use of an envelope sheet with a bond part on a basis of a sealing setting. The sealing system includes a transmission amount detector configured to detect an amount of transmission of the envelope sheet being conveyed on a conveyance path, a bond position calculator configured to calculate a first bond position where the bond part exists in the envelope sheet on a basis of the amount of transmission as detected by the transmission amount detector, and a judgment unit configured to judge whether the envelope sheet is conveyed a correct way round in a conveyance direction of the envelope sheet, from the first bond position as calculated by the bond position calculator and a second bond position based on the sealing setting.