1. An image forming device comprising: hardware performing operations to image data in an image memory; an image memory handler allocating the image memory to task threads and managing execution of the operations to the image data by the hardware; and a processing part managing as queues operation specifications to respective functions of the hardware demanded from the task threads, and managing execution of the operations to the image data by the hardware of the image memory handler, the processing part comprising: a common descriptor pool retaining a plurality of descriptors each of which issued in common as a descriptor that indicates a position of the image data being processed in the image memory; a united driver library providing functions called from the task threads, and returning a response corresponding to a command given from one of the task threads, to the task thread concerned; and a united driver managing operation specifications to device units corresponding to the respective functions of the hardware as queues, and managing execution of the operations to the image data by the hardware in accordance with the operation specification to the device units.