An image processing device reduces an image in a sub-scanning direction at a reduction factor of ½N (N>1) and includes one or more ASICs each of which performs image reduction at a reduction factor of 50%. When the one or more ASICs perform the image reduction processing in N stages each at the reduction factor of 50%, one of the ASICs performs an i-th stage of the image reduction processing at 50% (N−1≧i≧1) and stores resulting image data into a buffer memory that is equal in size to ½i of one band. Then, one of the ASICs performs an (i+1)-th stage of the image reduction processing at 50% on the image data acquired through the i-th stage of the image reduction processing and stores resulting image data into a buffer memory that is equal in size to ½i+1 of one band.