1. A viewing apparatus comprising: a transmission unit having light transparency; an identification information obtaining unit which obtains identification information to identify a user of the viewing apparatus; and a processor which performs: authentication processing which authenticates the user based on the identification information; judgment processing which judges whether a hidden image included in advance in the image can be viewed or not, based on a result of the authentication processing; and 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.