PURPOSE: To attain an efficient parallel operation at a low cost by referring to the attribute of each frame buffer memory, and temporarily saving the information of the frame buffer memory to a secondary storage device when the information which can be saved is present.

CONSTITUTION: The request of a job is issued from a computer 11 or the like, and the ensuring request of the frame buffer memory is issued. However, when the frame buffer memory size larger than the residual memory size is requested for the ensuring request of the frame buffer memory, whether or not the information which can be saved to a secondary storage device 13 is present in the already ensured frame buffer memory is checked. Then, when the information which can be saved is present in the frame buffer memory, a saving processing is operated. Thus, the new frame buffer memory can be ensured by the saving processing, and a pointer can be returned.