A sheet folding apparatus includes a fan folding unit that fan folds a folded-back sheet into a width or length of the regular size to file the sheet in the regular-size folder such that the sheet forms layers along a traveling direction of the sheet; first and second punching units that punch the filing hole in the sheet fan folded, respectively, into the width and the length of the regular size at the portion to be punched to file the sheet in the regular-size folder; and a sheet-processing setting unit that sets a fold mode of the fan fold and selects one of the first punching unit and the second punching unit for use in punching of the filing hole in the sheet based on a size and orientation of the delivered sheet.