In a sheet post processing apparatus having a folding device that carries out the process of book folding by folding either one sheet or a plurality of sheets at a time on the sheets discharge from the image forming apparatus, a sheet stacking section that successively stacks the booklet-folded sheets, and a control unit that controls the sheet post processing apparatus, said control unit controls said folding device so that said booklet folding process of said sheets is carried out at the portions of two folding lines which are almost parallel thereby obtaining a square-shaped back of booklet.