A lens unit is equipped with a lens, through which light from an object enters, and a lens barrel for holding the lens. An optical extending member is employed when observing a focal image of the lens with a microscope. A receiving portion having a receiving surface, for receiving a front end facet of the optical path extending member within the lens barrel that faces toward the lens, is formed at a portion of the lens barrel. A mounting portion positioned further rearward from a rear end facet of the optical path extending member, having a guide insertion aperture that extends in a direction perpendicular to the receiving surface and a mounting surface that extends in a direction parallel to the receiving surface is formed at the periphery of the guide insertion aperture is formed at a portion of the lens barrel.