This invention provides an image transmission device and an image display device capable of preventing easily the image data from being tampered with and of confirming the originality of the image data visually. More specifically, this invention provides an image transmission device and an image display device wherein: the image display device receives the coded-image data transmitted from the image transmission device and the encrypted mark coded-image data, and decrypts the mark image by using the hash value of the decoded image; the mark image connected with the image is output; when even a part of the image is tampered with, the coded-image data of the mark image which is decrypted by using the hash value as a key become different from the coded-image data of the mark image which is not tampered with; and consequently, the mark image finally achieved enables to judge visually and easily whether the image data are tampered with or not.