plural conveying paths including guide plates are arranged on both sides of a sheet in its thickness direction , being linked from an upstream side to a downstream side in a conveying direction of the sheet . a stopper is arranged in at least one of the conveying paths to make contact with a leading edge of the sheet conveyed through the conveying paths thereby stopping the sheet and forming a bending portion at the sheet . a pair of folding rollers is arranged between the conveying paths to nip the bending portion of the sheet to form a folding line at the sheet . an adjusting member is provided on the guide plates to adjust a space between the guide plates .