A tilt adjusting mechanism adjusts a tilt of at least one of a plurality of objective lenses so that they have the same inclination state. The tilt adjusting mechanism includes an aperture member to which an objective lens to be a target of the tilt adjustment is fixed, and a lens holder to which the aperture member is fixed from a slidable state for the tilt adjustment. As a surface for sliding movement of the aperture member with respect to the lens holder, there is provided a sliding surface that is a part of a spherical surface having a center that is a principal point of the objective lens or adjacent point thereof.