Sheet processing device, sheet laminator, image forming apparatus, and image forming system

A sheet processing device configured to separate a two-ply sheet in which two sheets are overlapped and partially bonded and sandwich a sheet medium between the two sheets, the sheet processing device comprising:

• a relay conveyance path through which the sheet medium is fed from an image forming apparatus connected upstream of the sheet processing device;
• a sheet loader configured to load at least one of the two-ply sheet and the sheet medium;
• and
• control circuitry configured to perform a mixing mode to feed the two-ply sheet or the sheet medium from the sheet loader,
• wherein, when the control circuitry determines that a first conveyed sheet from a plurality of sheets is the two-ply sheet and a sheet sensor does not detect the two-ply sheet in a separated state on a conveyance path in the sheet processing device during execution of the mixing mode, the control circuitry is configured to perform sheet processing to separate the first conveyed sheet determined as the two-ply sheet.