An image forming apparatus which is capable of accurately performing a video count process based on image data and thus maintaining the quality of images even where DMA transfer processing is used. An image forming device performs a process using a consumable material. A first controller controls the image forming device. A transfer controller provides control to transfer externally input image data to the first controller. A second controller performs a count process for counting an amount of the consumable material to be consumed based on the image data transferred by the transfer controller. The second controller disables the count process without stopping the transfer by the transfer controller when the process performed by the image forming device is interrupted.