in fig2 so that the sheet trailing end pressing member 98 abuts against the upper surface of the lifted sheet bundle to press the sheet bundle with this pressing action even if there is any curled sheet the proper sheet surface height is maintained when the sheet surface height detecting sensor 97 shown in fig1 leaves the sheet bundle in response to the lowering movement of the stack tray 80 the photo interrupter 132 is turned off however when the sheet surface height detecting sensor 97 is pushed again by the upper surface of the sheet bundle in response to the lifting movement of the stack tray 80 as shown in fig2 the sheet surface height detecting sensor 97 turns the photo interrupter 132 on thereby detecting the sheet surface height after the detection of the sheet surface height the stack tray 80 is lifted by a predetermined amount for example about 12 mm and then is stopped in this case the sheet trailing end pressing member 98 presses the upper surface of the sheet bundle with proper pressure control of the lifting lowering movement of the stack tray 80 is effected by a control device 150 including a cpu 151 fig2 in this way by arranging the sheet surface height detecting sensor 97 in the vicinity of the sheet trailing end pressing member 98 proper detection of the sheet surface height and the pressing operation of the sheet trailing end pressing member 98 with proper pressure can be realized further since the trailing end of the sheet bundle bundle discharged by the bundle discharging belt 60 is held by the sheet trailing end pressing member 98 the sheet bundle can be bundle discharged without pushing out the already stacked sheet bundle s when the sheet bundle discharged onto.