PURPOSE:To perform reliable detection of the presence of sheets in a containing box without being influenced by a state, e.g., sheet powder, dust, and bending of sheets, by a method wherein first and second optical sensors to detect the presence of sheets accumulated on a press plate are mounted in a state that the respective optical axes are positioned in an oblique state. CONSTITUTION:During the starting of an accumulation motion, a press plate 2 is raised to the given position of the upper surface of sheets to ensure a space necessary to the accumulation motion of sheets 3. With this state, the one end part of a guide plate 14 is pressed by means of the upper surface of the press plate 2, and the guide plate 14 is rotated clockwisely. The position of the other end of the guide plate 14 is detected by means of an emission element 16 and a collection element 17 of an optical sensor. Uptake rollers 11 and 12 of a sheet accumulating means are rotated in the direction of arrow marks (a) and (b), the sheets 3 are introduced one by one from a preceding process toward the direction of an arrow mark (c), and the sheets 3 are laminated and accumulated on the press plate 2 through the guide plate 14. The position of the upper surface of the sheets 3 is detected through the guide plate 14 by means of the collection element 16 and the emission element 17.