1. An image processing apparatus that superimposes additional information on image data, the apparatus comprising: a region setting unit configured to set, in the image data, a main region in which the superimposing intensity for superimposing the additional information is constant, and a superimposing intensity change region in which the superimposing intensity for superimposing the additional information is not constant, a superimposing intensity setting unit configured to set the superimposing intensity according to the position of a target pixel, and a superimposing unit configured to superimpose 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 constant when the position of the target pixel is in the main 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 superimposing intensity change region.