An image forming apparatus having: image forming means for visualizing image data read out by image reading means and forming it onto transfer materials; post-processing means for executing a post-process including a stapling process, a folding process, and/or a sorting process in order to execute a book-binding process to the transfer materials on which the image data has been formed; and a trimmer for executing a trimming process to a bundle of the transfer materials subjected to the binding process by the post-processing means, wherein when the trimming process by the trimmer is executed, the image forming means sets a non-image forming area including a processing portion to which the trimming process is executed and a valid image forming area excluding the non-image forming area onto the transfer material and forms the image data read out by the image reading means into the valid image forming area of the transfer material.