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 that when the original image is reduced, producing a reduced image, the mosaic in the reduced image fails to conceal the object in the reduced image; and a re-mosaicking means for, in a case where it has been judged by said judgment means that the mosaic in the reduced image fails to conceal the object, re-mosaicking the to-be-concealed object of said original image with blocks having a re-mosaic block size that when reduced conceal the object in the reduced image.