PURPOSE: To rotate easily image data with a simple mechanism, by providing an image data storage memory and a word address generating circuit which supplies addresses where image data are written in word units.

CONSTITUTION: In repsect to an image data storage memory and a word address generating circuit which supplies addresses where image data are written in word units, for example, the output of a write word address generating circuit 2a is selected by a multiplexer 4. Next, contents of m-number words from word 0 to word (m-1) of an image data memory 1 are written in word units in the same position of an image data storage memory 5a. After writing all words, the multiplexer 4 is switched to a read bit address generating circuit 3, and bit (n-1) of each word is read out for rotation at -90°, and bit 0 of each word is read out for rotation at +90°, thus rotating image data.