1. An image processing device comprising: a means for judging whether or not a mosaic block size of a mosaic arranged in a to-be-concealed object of an original image is a size such that reducing the original image allows the mosaic to disappears visually; and a re-mosaicking means for, in a case where it has been judged by said judgment means that the mosaic arranged in said to-be-concealed object disappears visually, re-mosaicking the to-be-concealed object of said original image with blocks having a re-mosaic block size such that the mosaic does not disappear visually even though said original image is reduced.