An OCT probe has a sheath to be inserted into a subject; and an optical system within the sheath, for changing the direction of light which propagates from a light source through an optical fiber to irradiate the light onto the subject through a transparent portion of the sheath, and for reflecting the light beam, which is reflected by the subject, to guide the light into the optical fiber. A light output surface, for causing the light to be output from the reflecting surface in a direction obliquely inclined with respect to the inner surface of the sheath, and a reflected light sheath entrance preventing portion, for preventing light reflected by the inner surface of the sheath from entering the optical fiber, are provided in the optical system. The reflected light sheath entrance preventing portion may be formed by providing a cut planar surface portion on the lens.