1. An image processing apparatus that superimposes additional information on image data, the apparatus comprising: a region setting unit that sets, in the image data, a first region and a second region on which the additional information is respectively superimposed with a constant superimposing intensity, and a third region on which the additional information is superimposed with a non-constant superimposing intensity, wherein the second region is between the first region and the third region; a superimposing intensity setting unit that sets the superimposing intensity according to a position of a target pixel; and a superimposing unit that superimposes the additional information on image data of the target pixel using the superimposing intensity set by the superimposing intensity setting unit, the superimposing intensity being set to be constant when the position of the target pixel is in the first region or the second region and the superimposing intensity being changed according to the position of the target pixel when the position of the target pixel is in the third region, so that the superimposing unit superimposes the additional information on the first and second region with the constant superimposing intensity and superimposes the additional information on the third region with the superimposing intensity set by the superimposing intensity setting unit, wherein a pattern as the additional information for superimposing on the second region is different from a pattern as the additional information for superimposing on the first region and is different from a pattern as the additional information for superimposing on the third region.