A printer control apparatus can be applied to an image forming apparatus in which a plurality of types of readers and a plurality of types of printers can be connected to a single controller, and which enables efficient transfer of image data from the reader to the printer via an image memory of the controller. The controller receives image data from a reader and temporarily stores the same in an image memory, and transmits the stored image data to a printer. The controller acquires a reader processing speed from the reader and a printer processing speed from the printer, and determines, while the image data is being written to the image memory, timing for transmitting the image data written in the memory to the printer according to a difference between the reader processing speed and the printer processing speed and a sub-scan size of the image data.