To prevent an operation mode which is contrary to the intention of a user from being selected.

In the optical device provided with a detecting means for the line of sight to detect the position of the line of sight of the user viewing an observation surface by picking the eye-ball image of the user and a photometric means to measure brightness within the observation surface and having a function to select a specified operation mode among several operation modes, the selection of the specified operation mode by the detected result of the detecting means for the line of sight is not performed (S104→S1l4) in the case that a photometric value obtained by the photometric means is equal to or lower than a specified value.