Light emitted from the light source unit is divided into measuring light and reference light. An optical path length of the measuring light or the reference light which has been divided by the light dividing means is adjusted. Interference light of the reflected light and the reference light is detected and a tomographic image of the object is obtained on the basis of the detected interference light. The optical path length is adjusted by a reflecting mirror which reflects the measuring light or the reference light radiated from the optical fiber, a first lens which is disposed between the reflecting mirror and the optical fiber and a second lens which collects the measuring light or the reference light made parallel by the first lens on the reflecting mirror and makes parallel the measuring light or the reference light reflected by the reflecting mirror.