An image forming apparatus which is capable of reducing a load on a data bus therein when controlling an image reading apparatus depending on its image processing capability. When acquired identification information about the image forming apparatus is included in a clock control table 600, a clock frequency for controlling hardware of the image reading apparatus in accordance with the clock control table 600, while setting the clock frequency therefor to a default setting of the clock control table 600 when the acquired identification information is not included in the clock control table 600, wherein a minimum value of clock frequencies in the clock control table 600 is set as the default setting of the clock control table 600.