A control device executes: either a first conveyance change control or a second conveyance change control; and a retraction control when an operation device receives an interruption operation during execution of a specified number stack control. In the first conveyance change control, a sheet conveyance device of a target unit is changed to a relay conveyance state, and the sheet conveyance device of another coupled unit that is present downstream of the target unit in a main conveyance direction is changed to a stack conveyance state. In the second conveyance change control, the sheet conveyance device of another coupled unit that is present upstream in the main conveyance direction is changed to the stack conveyance state. In the retraction control, a lift mechanism of the target unit is caused to execute a mount operation by which a stack tray is moved down to a mount position on a carriage portion.