This invention provides a technique of preventing a collision between an original document and a printing material on a conveyance path which is partially shared for conveyance of them in an image forming apparatus. The image forming apparatus of one aspect of this invention, which includes a printing unit configured to convey a printing material via at least part of a conveyance path used for original document reading processing, and execute printing processing of an image on the printing material, executes if the original document reading processing is not being executed when the printing processing of the image on the printing material is instructed, the printing processing without waiting for completion of the reading processing, and executes, if the original document reading processing is being executed, the printing processing upon completion of the reading processing.