1. A viewing apparatus through which an image is viewed, the apparatus comprising: a transmission unit having light transparency; an identification information obtaining unit which obtains identification information from a user of the viewing apparatus in a state in which the user is using the viewing apparatus, to identify the user of the viewing apparatus, wherein the identification information obtaining unit is arranged to face a side of the user of the viewing apparatus; a user information obtaining unit which obtains user information from the image, and which is arranged to face a side opposite the user of the viewing apparatus, wherein the user information indicates a user who has permission to view a hidden image included in advance in the image; and a processor which performs: (i) authentication processing which authenticates the user of the viewing apparatus based on the identification information obtained from the user of the viewing apparatus; (ii) judgment processing which, if the user of the viewing apparatus is authenticated in the authentication processing, judges whether the hidden image included in advance in the image can be viewed by the user of the viewing apparatus or not, wherein the judgment processing judges that the hidden image can be viewed by the user of the viewing apparatus when the user of the viewing apparatus who was authenticated in the authentication processing is included in the user information obtained from the image by the user information obtaining unit; and (iii) adjustment processing which adjusts light passing through the transmission unit to switch whether the hidden image can be viewed or not, based on a result of the judgment processing.