An image forming apparatus comprising: an image forming unit that forms an image on a sheet; and a sheet folding unit that is provided in a body of the image forming apparatus and that performs folding processing on a sheet, characterized in that the sheet folding unit comprises: a first sheet conveying unit configured to hold a part of a sheet to apply conveying force to the sheet; a second sheet conveying unit (13, 14) disposed downstream of the first sheet conveying unit in a sheet conveying direction, and configured to hold another part of the sheet to apply conveying force to the sheet; a folded part forming unit (12, 13) configured to form a folded part on the sheet by nipping a curved portion formed on a part of the sheet between the first sheet conveying unit and the second sheet conveying unit (13, 14), the curved portion being formed by holding a part of the sheet by the first sheet conveying unit and applying conveying force to the sheet to reverse the sheet upstream in the sheet conveying direction by the second sheet conveying unit (13, 14); and a sheet leading end detection unit configured to detect a leading end of a sheet and is disposed downstream of the second sheet conveying unit (13, 14) in the sheet conveying direction or at almost same position as the second sheet conveying unit (13, 14), wherein the sheet is conveyed based on a detection timing of the sheet leading end detection unit such that the leading end of the sheet protrudes by a predetermined protrusion amount from the second sheet conveying unit (13, 14) immediately before applying conveying force to the sheet to reverse the sheet. an image forming unit that forms an image on a sheet; and a sheet folding unit that is provided in a body of the image forming apparatus and that performs folding processing on a sheet, characterized in that the sheet folding unit comprises: a first sheet conveying unit configured to hold a part of a sheet to apply conveying force to the sheet; a second sheet conveying unit (13, 14) disposed downstream of the first sheet conveying unit in a sheet conveying direction, and configured to hold another part of the sheet to apply conveying force to the sheet; a folded part forming unit (12, 13) configured to form a folded part on the sheet by nipping a curved portion formed on a part of the sheet between the first sheet conveying unit and the second sheet conveying unit (13, 14), the curved portion being formed by holding a part of the sheet by the first sheet conveying unit and applying conveying force to the sheet to reverse the sheet upstream in the sheet conveying direction by the second sheet conveying unit (13, 14); and a sheet leading end detection unit configured to detect a leading end of a sheet and is disposed downstream of the second sheet conveying unit (13, 14) in the sheet conveying direction or at almost same position as the second sheet conveying unit (13, 14), wherein the sheet is conveyed based on a detection timing of the sheet leading end detection unit such that the leading end of the sheet protrudes by a predetermined protrusion amount from the second sheet conveying unit (13, 14) immediately before applying conveying force to the sheet to reverse the sheet. a first sheet conveying unit configured to hold a part of a sheet to apply conveying force to the sheet; a second sheet conveying unit (13, 14) disposed downstream of the first sheet conveying unit in a sheet conveying direction, and configured to hold another part of the sheet to apply conveying force to the sheet; a folded part forming unit (12, 13) configured to form a folded part on the sheet by nipping a curved portion formed on a part of the sheet between the first sheet conveying unit and the second sheet conveying unit (13, 14), the curved portion being formed by holding a part of the sheet by the first sheet conveying unit and applying conveying force to the sheet to reverse the sheet upstream in the sheet conveying direction by the second sheet conveying unit (13, 14); and a sheet leading end detection unit configured to detect a leading end of a sheet and is disposed downstream of the second sheet conveying unit (13, 14) in the sheet conveying direction or at almost same position as the second sheet conveying unit (13, 14), wherein the sheet is conveyed based on a detection timing of the sheet leading end detection unit such that the leading end of the sheet protrudes by a predetermined protrusion amount from the second sheet conveying unit (13, 14) immediately before applying conveying force to the sheet to reverse the sheet.