In a sheet folding apparatus for forming first folding and second folding in a sheet, to enable the sheet to be folded in an accurate fold position in second folding, when the front end of the sheet which is first folded in a nip position Np1 of a first folding roller pair and transported strikes a stopper member 86, a guide member 54 guides a fold to undergo second folding of the first-folded sheet to a nip position Np2 of a second folding roller pair 101, and second folding is formed.