An image forming system includes an image forming unit configured to form an image on a sheet, a reversing portion in which a front side and a back side of the sheet are to be reversed in order to form an image on a surface opposite to a surface of the sheet on which the image has been formed by the image forming unit, a duplex-printing conveying path for use in conveying the sheet having the front side and the back side reversed by the reversing portion to the image forming unit, a registration unit configured to correct a skew of the sheet conveyed to the duplex-printing conveying path, and an image reading sensor configured to read the image formed on the sheet having the skew corrected by the registration unit in the duplex-printing conveying path.