PURPOSE: To realize rotating processing of image data of a bit map form even in the system whose memory capacity is small.

CONSTITUTION: The device is provided with an image data division means 12 dividing original image data into plural blocks, an image data reduction means 13 reducing the image data of one block at a prescribed ratio, an image data rotation means 14 applying rotation processing to the reduced image data, and an image data magnification means 15 applying magnification processing to the image data subject to rotation processing to be restored to the original size, and also with a buffer 11A whose capacity is one block of the original image data, a buffer 11B whose capacity is equal to the reduced image data of one block, and a buffer 11C whose capacity is equivalent to the capacity of the image data subject to reduction processing of the original image data, and with a memory capacity management means 16 deciding the allocation of the share of the buffers 11A-11C in the memory based on the capacity of the original image data and the memory size available for the image rotation processing.