An image I/O apparatus has a paper feeding direction specifying control unit 125 which is constructed in a manner such that when a plurality of paper sizes exist mixedly in one image forming job, on the basis of paper mixture information which is set on a printer driver of a PC/WS 11 and shows which paper sizes exist mixedly in the one image forming job, the paper feeding direction for performing a paper feed in the vertical direction or a paper feed in the lateral direction to the paper which can be fed in both of the vertical direction and the lateral direction is specified.