A pseudo random number generating circuit forms uniform distribution random number noises. An insertion noise data forming unit receives the uniform distribution random number noises, center-distributes them around 0 as a center, further reads out output correction values of every recording device from an LED characteristics variation data memory, center-distributes them on the basis of the output correction values, and outputs them. A noise inserting circuit receives an output of the insertion noise data forming unit and adds it to input image data. The occurrence a stripe-shaped concentration variation on an output image is prevented, so that a good output image is obtained.