Provided is a sheet processing apparatus for performing a bookbinding process by folding a sheet stack, including: a folding roller pair including a first roller and a second roller brought into press contact with the first roller to be contacted/separated with/from the first roller, for conveying the sheet stack while folding the sheet stack; a plurality of urging members capable of urging the second roller in a direction of the first roller; and an urging mechanism for changing a number of the urging members for urging the second roller according to a thickness of the sheet stack to be folded.