A sheet processing apparatus which vertically conveys a sheet and performs sheet processing, the sheet having a folded part at a middle portion thereof and being in a vertical state with the folded part up and an edge part down, and the apparatus including: a supporting section which supports the folded part of the sheet from below; a sandwiching section which sandwiches both sides of the sheet supported by the supporting section; and a control section which controls the sandwiching section to execute a first sandwiching operation of sandwiching the sheet, to release the first sandwiching operation, and thereafter to execute a second sandwiching operation of sandwiching the sheet again.