A flexible shaft driven to rotate is inserted through a transparent sheath having pliability. By a fiber inserted through the inside thereof, low-coherence light is guided and is made to exit to a living-body tissue side which is an observation target through a lens and a prism forming an exit and entrance portion at the tip portion. Subsequently, the light reflected on the living-body tissue side is guided in order to produce an image. In that case, a positioning member for keeping the exit and entrance portion and the living-body tissue at a proper distance is formed at the tip portion of the sheath or the tip portion of an endoscope through which an optical probe is inserted and, therefore, a stable tomogram image can be produced.