1. An image forming apparatus comprising: image forming means for visualizing image data 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 binding process to the transfer materials to which said 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 said post-processing means, wherein said image forming means has means for, when the trimming process by said trimmer is executed, setting a non-image forming area including a processing portion to which said trimming process is executed and a valid image forming area excluding said non-image forming area onto the transfer material and forming said image data into said valid image forming area on said transfer material.