A sheet processing apparatus is disclosed which includes front and back trailing-end reference fences that are movable and on which the sheet trailing-end abuts to align the sheet trailing-end; a side-stitching stapler that moves along the sheet trailing-end and binds a bundle of sheets aligned; a length-direction moving mechanism that moves the trailing-end reference fences in a sheet conveying direction; and a position changing unit that drives the length-direction moving mechanism to change the position of one of the trailing-end reference fences in the sheet conveying direction relative to the position of the other one. The front and back trailing-end reference fences are moved to change binding positions such that the distances from the sheet trailing-end to staple positions are made equal in front-side binding and in back-side binding.