In an image forming apparatus of the present invention, when performing one-side printing during a predetermined period after two-side printing, a change in a carrying velocity of a post-process apparatus is prevented and thus recording paper having passed through a fixing section is carried and output at a velocity that is lower than a velocity for normal one-side printing and that is identical with a velocity for two-side printing. Consequently, it is possible to prevent dropping of productivity of the image forming system due to a time necessary for changing the carrying velocity of the post-process apparatus and to increase an operating ratio of the image forming system, even if the post-process apparatus is designed to completely stop the carrying roller etc. when changing the carrying velocity.