Each of the sheets is conveyed by a horizontal conveyance unit in a state of being overlapped with a next sheet from above. Each of the sheets is conveyed out of the horizontal conveyance unit in a state of being folded along a fold line with the fold line oriented upward, the fold line extending in a conveyance direction of the horizontal conveyance unit. An oblique conveyance unit receives the sheet from the horizontal conveyance unit and successively conveys the sheets in an oblique downward direction. A sensor is arranged to detect passage of the sheets at a position where steps pass. Each of the steps is formed by two sheets adjacent to each other diverted to the oblique downward direction by the oblique conveyance unit.