1. An image processing apparatus, comprising: a model that represents at least the shape of a predetermined structure by a statistical characteristic amount obtained by performing predetermined statistical processing on a plurality of images representing the structure, and a weighting parameter for weighting the statistical characteristic amount according to an individual characteristic of the structure; a first parameter obtaining means for obtaining a value of a weighting parameter for representing the structure in an input image by applying the structure in the input image to the model; an image retouching means for retouching a portion of the structure in the input image; a second parameter obtaining means for obtaining a value of the weighting parameter for representing the portion of the structure in the input image before the retouching by applying the portion of the structure in the pre-retouching input image to the model; a third parameter obtaining means for obtaining a value of the weighting parameter for representing the portion of the structure in the input image after the retouching by applying the portion of the structure in the post-retouching input image to the model; a fourth parameter obtaining means for obtaining a value of the weighting parameter for representing the entire portion of the structure after the retouching is reflected to the entire portion of the structure by the model based on the value of the weighting parameter obtained by the first parameter obtaining means, and the variation from the value of the weighting parameter obtained by the second parameter obtaining means to the value of the weighting parameter obtained by the third parameter obtaining means; and an image reconstruction means for reconstructing an output image representing the structure after the retouching is reflected to the entire portion of the structure by substituting the value of the weighting parameter obtained by the fourth parameter obtaining means to the model.