A focusing unit (16) can be inserted into and removed from an illumination light path by rotating around a supporting portion (16c). An index light source (15), a green LED, is attached to the bottom of a base (16a). A transparent acrylic-resin disk (16b) is fixed to the base. A deflection prism (13) is attached to the bottom of the disk. A beam from the light source enters the deflection prism. A focus index forming portion (14) is provided on the top of the disk. In the center of a light-shielding portion (14a) is formed a slit-like opening (14b), under which split prisms (14c, 14d) are provided. Deflected by the prisms, the beam from the light source is split into two, right-half and left-half, directional beams, and combined with an illumination optical system, in the form of two slit images, at the opening.
