It is made possible to prevent decrease in the density due to the image output characteristic of the image output unit in the subscanning direction by means of a method according to the present invention capable of real-time processing without causing the circuit capacity and the memory to be increased. In an image input unit 100, the input image data is acquired so that its time series direction becomes a direction indicated by an arrow 42 corresponding to the subscanning direction of an image output unit 300. The input image data from the image input unit 100 is corrected in accordance with the image output characteristic of the image output unit 300 in the subscanning direction. The image data after the correction is rotationally processed so that its time series direction becomes a direction indicated by an arrow 41 corresponding to the main scanning direction of the image output unit 300. The image data after the rotary processing is sent to the image output unit 300.