An image forming apparatus transfers plot data stored in a first storing part of a side of a controller to a side of a writing engine. A second storing part temporarily stores the plot 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 plot 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 plot data from the first storing part to the second storing part, reading of the plot data from the second storing part is started for starting image forming operation.