1. An image forming apparatus including a separation mode for permitting a transfer medium, at time of a monochrome print, to make contact with only a monochrome image forming unit and separating the transfer medium from a plurality of remaining image forming units and a contact mode for permitting the transfer medium to make contact with all the plurality of image forming units and enabling the modes to be switched, comprising: judging means for judging, based on image information, whether an image to be printed is a monochromatic image or a full color image; operation mode deciding means for deciding which mode is used to print the image, the contact mode or the separation mode; printing means for printing the image in the decided mode, the operation mode deciding means, at the beginning of a job, deciding the separation mode for the monochromatic image and the contact mode for the full color image and after a predetermined condition being satisfied on the basis of actual results of the page printed already in the job, deciding the contact mode for both monochromatic images and full color images; and setting means for setting a reference value for judging success or failure of the predetermined condition.