This invention relates to a sheet postprocessing apparatus which includes a first crease forming unit for forming a first crease on a sheet in its width direction, a second crease forming unit for forming a second crease parallel to the first crease, a first abutting member against which the leading end of a sheet introduced into the first crease forming unit abuts to position the sheet, a second abutting member against which the first crease abuts to position the sheet, a first driving mechanism for driving the first abutting member, a second driving mechanism for driving the second abutting member, and a control device for controlling the first crease forming unit, first driving mechanism, second crease forming unit, and second driving mechanism, and makes one folding process selected from a center folding process, inward three-fold process, and Z-fold process executable in the same sheet convey path.