An image forming apparatus transfers image data stored in a first storing part of a side of a first control part to a side of a writing engine. A second storing part temporarily stores the image data from said first storing part. When data stored in the second storing part reaches a data amount to be previously stored in order that continuous transfer of the image data to the side of the writing engine from the second storing part is available at a predetermined transfer rate, even when a predetermined delay occurs in a transfer operation of the image data from the first storing part to the second storing part, reading of the image data from the second storing part is started for starting image forming operation.