A duplex image forming device includes an image forming device and a reversible transportation unit. The image forming device includes a paper transportation path, a paper feed unit which transports a paper to the paper transportation path, a printing unit which prints an image onto the paper, and a discharge tray where the paper printed with the image is discharged. The reversible transportation unit includes a main body frame having a contacting surface that contacts against the side of the image forming device, a reversible transportation path which transports-in the paper from a downstream side of the printing unit and transports-out the paper to an upstream side of the printing unit in the paper transportation path, transportation rollers for transporting the paper through the reversible transportation path, and a motor which drives the transportation rollers and is disposed protruding outward from the contacting surface of the main body frame so that at least a part of the motor is disposed in the image forming device when the reversible transportation unit is inserted into the image forming device.