A sheet processing system includes a sheet processing device and a lamination processing device. The sheet processing device includes a sheet separator that separates a two-ply sheet and inserts an inner sheet between the two-ply sheet to form a first enclosed two-ply sheet. The lamination processing device bonds the first enclosed two-ply sheet, and bonds a second enclosed two-ply sheet in which the inner sheet is inserted outside the sheet separator. The sheet processing system includes a relay conveyor that conveys the first enclosed two-ply sheet from the sheet separator to the lamination processing device attachable to a sheet feeder to directly feed the second enclosed two-ply sheet to the lamination processing device. The sheet processing device includes a cord holder to hold the power cord of the lamination processing device at a position where feeding and ejecting the two-ply sheet after bonding the two-ply sheet are not hindered.