A sorting machine (1) for sorting flat articles (2) on edge, which machine includes an upstream conveyor (10), and a downstream conveyor (30) that conveys the flat articles (2) by nipping them, the upstream conveyor (10) and the downstream conveyor (30) being separated by detector means (100) including an intermediate conveyor (20) provided with intermediate belts (22) that convey said flat articles (2) by nipping them, and that are associated individually with suction means (24, 25) pressing any flat article (2) against one or the other of said intermediate belts (22), image capture means (26) for capturing images of the bottoms of said flat articles (2) passing between the intermediate belts (22), and analysis and processing means (4) for analyzing and processing said images to detect any bunching of said flat articles (2).