The adapter correcting for glass thickness includes an adapter main body, a plane parallel plate, and a retainer. The adapter main body includes an attacher/detacher capable of attaching and detaching with respect to an adapter connecter on the lens barrel; a tubular accommodator extending from the attacher/detacher toward the objective along an optical axis of the field lens; and a tongue provided on the attacher/detacher side of the accommodator. The plane parallel plate includes two mutually parallel flat surfaces and is inserted into the accommodator such that the two flat surfaces are orthogonal to the optical axis of the field lens. The retainer is attached to an end of the accommodator closest to the objective and holds the plane parallel plate between the retainer and the tongue. The accommodator is formed to have a tubular length sufficient to insert at least two plane parallel plates.