An optical unit for liquid immersion change attachable to a dry-type image-forming optical system for forming the image of an object through a gas and used for changing the dry-type image forming optical system to a liquid-immersed-type image-forming optical system for forming the image of the object through a liquid. This optical unit comprises at lease one optical member interchangeable with an optical member disposed on the image side of the optical surface that has a predetermined refractive power and is disposed closest to the image side, among the optical surfaces having a refractive power and arranged in the dry-type image-forming optical system. By interchanging the optical member of the dry-type image-forming optical system with the at lease one optical member, the degradation of the image-forming characteristic of the image forming optical system caused by the change of the medium kind on the image side of the image-forming optical system from a gas to a liquid is reduced.
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