PURPOSE: To automatically bind sheets accumulated in each bin, by providing a notch in an appropriate position of an edge, which serves to true up the accumlated sheets, of each bin in a sorter and mounting a motor-driven stapler so that it is vertically movable along the outside of the edge with the notch for trueing up the sheets.

CONSTITUTION: A sorter, in which sheets are supplied through a sheet guide 2, is constituted such that bins 7 are located in a multistage, and a distance between the bins 7 is variable in synchronism with charging of the sheets by means of an indexer 6 and through operation of ratchet blocks 12 and 13. In which case, in order to staple the accumulated sheets in each bin 7, an automatic stapling device 22 is vertically movably located along the row of the bins 7. The device 22 is constituted such that a stapler 25 is horizontally movably mounted to an elevating block 23 elevating through rotational movement of a screw shaft 24. Furthermore, a notch 27 with a given area is provided in a part where the stapler 25 of the bin 7 moves in, so that a part, situated on the notch 27, of the accumulated sheets can be stapled.