An overlapped mail detection apparatus and method are disclosed. Since the postcard is shorter than the letter, the change in length is not sometimes detected even when the postcard is displaced. In the case where the exterior of the letter is deformed outward and an overlap is detected from the rise of the tail end of the letter after passing a guide, a thin letter or the like low in rigidity is hard to detect due to the slow rise after deformation in the guide. In view of this, a separator has a bent conveyance path for deforming the exterior of the letter outward. The conveyance surfaces constituting the inner and outer sides of the bent portion of the separator are spaced appropriately from each other. A detector detects the state of the letter surface in the neighborhood of the outer conveyance surface with which the forward end of the letter first comes into contact. An overlap of letters is judged from the result of detection by the detector.