To stack sheets securely when a front and rear or left and right thickness of the sheets to be processed is different and the thickness of binding part in a booklet type product is different as compared with the that of a surface except the binding part.

A stacker device comprises a mounting base on which processed sheet are sequentially stacked and a sheet delivery part for the sheet stacked on the mounting base by supplying the processed sheets obliquely upper of the mounting base and a first and a second sheet mounting level detecting devices 38a, 38b for detecting mounted height of the mounted sheets separately front and rear or left and right of the sheet and also the mounting base has a mounting base slanted drive part 37 for moving up and down one of the front and rear and the left and right and a mounting base up and down driving part 39 for driving the whole mounting base up and down. And a control part for driving the mounting base up and down driving part 39 by a detecting signal from the first and second sheet mounting level detecting device 38a, 38b when the sheet is supplied from the sheet delivery part to drive the mounting base downward and also driving a mounting base obliquely driving part 37 to control lowering of the mounting base while keeping a sheet mounting surface approximately horizontal.