PURPOSE: To shorten the whole time for forming images by performing at a time extending and generating processes of a next bit map image for a memory area in memory means whose transfer is completed when extended and generated bit map images are to be transferred to image formation processing means.

CONSTITUTION: Tt1 is a transfer time of a bit map image extended on a first page in a video RAM 745 and is a fixed time dependent on size of each page in the video RAM 745. When a transfer of the first page proceeds, an extension of image information of a second page corresponding to a memory area whose transfer is completed is started. At the time that the transfer of the first page is completed and a transfer preparation time Te for a next page has elapsed, the extension for the second page has been completed, so that the transfer of the second page is possible. When the transfer of the second page proceeds, an expansion of a third page is started for the memory area of the second page in the video RAM 745 whose transfer is completed. At the time that the transfer of the second page is completed and the transfer preparation time Te for the next page has elapsed, the expansion of the third page is not completed, so that the transfer of the third page will be started after an elapse a time Tw where the expansion for the third page completes.