To appropriately reflect the influence by the ambient light of an image display device on an image displayed by the image display device.

A sensor signal processing section 11 calculates vectors v1 to v4 indicating the positions where the maximum intensity of the images 31 to 34 obtained by two-dimensional optical sensors 3 to 6 and parameters c1 to c4 indicating the deviation of the optical intensity distribution in the images 31 to 34. An illumination condition calculation section 12 calculates the intensity distribution (contrast) CL of the light on the display screen of the image display device 1 by using the parameters c1 to c4. The condition calculation section 12 calculates the vectors toward the position of a light source 2 from the position of each object. An image processing 13 creates a shadow image based on the contrast cL calculated in the condition calculation section 12.