A sheet folding apparatus for folding a set of sheets into two at a center thereof, includes: a sheet number recognition section that recognizes the number of sheets in the set; a pair of rotating rollers pressed against each other to form a nip portion; a blade member retractably moves with respect to the nip portion; and a blade member control section that controls the blade member to move toward the nip portion to push the set of sheets in the nip portion at a depth corresponding to the number of sheets recognized by the sheet number recognition unit.