A reference member which regulates a position of one side end of a sheet along the sheet conveying direction is provided at one side of a sheet conveying path in the width direction so as to follow the sheet conveying direction and to be rotatable about its downstream end portion in the sheet conveying direction. Then, one side end of the sheet is skew-conveyed toward the reference member by a plurality of pair of skew-conveying rollers which is arranged in the sheet conveying path along the sheet conveying direction. Then, when forming an image on both faces of the sheet, a control portion rotates the reference member so that the sheet is displaced to the center side in the sheet conveying path compared to the case of forming an image on one face of the sheet.