Disclosed is an image forming apparatus capable of increasing double-side-printing speed. The image forming apparatus comprises a paper transfer unit provided with a plurality of paper transfer sub-units for feeding a printing paper to a double-side-printing paper transfer path for reversing a printing paper. The paper transfer direction is switched by a switching unit selectively engaged with the respective paper transfer sub-units. According to the embodiments of the present invention, because the paper transfer sub-units always rotate in their own directions, it is possible to prevent the delay of printing caused in the process of switching reverse and forward rotations of a conventional paper transfer unit for reversing a paper, whereby the double-side-printing speed can be increased.