An image forming apparatus which is capable of bringing processing devices, such as sheet feed devices and post-processing devices connected respectively to the image forming apparatus, into operable states in timing optimal for carrying out an image forming job, thereby improving productivity and operation rate of the image forming apparatus as well as saving energy. A print job is analyzed to determine at least one sheet feed device and at least one post-processing device to be used in executing the print job, and states of the determined devices are checked to determine at least one of the sheet feed device and the post-processing device to be caused to start a preparatory operation in advance before the print job is executed. Power supply to the at least one of the sheet feed device and the post-processing device is turned on in timing such that the preparatory operation is completed in time for the start of execution of the print job.