An adapter (100; 100A) correcting for glass thickness configured to be attachably and detachably provided to a field lens comprising a lens barrel having a tubular shape and a plurality of lenses accommodated within the lens barrel, the field lens using the plurality of lenses to correct aberrations due to a glass plate having a specified thickness being interposed between the lens barrel and an objective, the adapter (100; 100A) correcting for glass thickness comprising: an adapter main body comprising: an attacher/detacher configured to be capable of attaching and detaching with respect to an end of the lens barrel closest to the objective; an accommodator having a tubular shape extending from the attacher/detacher toward the objective and axially centered on an optical axis of the field lens in a state where the adapter (100; 100A) for correcting for glass thickness is attached to the field lens; and a tongue provided on the objective side of the accommodator and projecting inward from an inner circumferential surface of the accommodator tube; at least two translucent plane parallel plates each having two mutually-parallel flat surfaces, wherein it is possible to insert one of the plane parallel plates into the accommodator and also to insert two combined plane parallel plates into the accommodator such that the respective two flat surfaces are orthogonal with respect to the optical axis of the field lens in a state where the adapter (100; 100A) for correcting for glass thickness is attached to the field lens; and a retainer attached to an end of the accommodator closest to the objective, the retainer holding the plane parallel plates between the retainer and the tongue in a state where the plane parallel plates are positioned within the accommodator, and wherein the accommodator has tubular length configured to accommodate at least the two plane parallel plates therein. an adapter main body comprising: an attacher/detacher configured to be capable of attaching and detaching with respect to an end of the lens barrel closest to the objective; an accommodator having a tubular shape extending from the attacher/detacher toward the objective and axially centered on an optical axis of the field lens in a state where the adapter (100; 100A) for correcting for glass thickness is attached to the field lens; and a tongue provided on the objective side of the accommodator and projecting inward from an inner circumferential surface of the accommodator tube; an attacher/detacher configured to be capable of attaching and detaching with respect to an end of the lens barrel closest to the objective; an accommodator having a tubular shape extending from the attacher/detacher toward the objective and axially centered on an optical axis of the field lens in a state where the adapter (100; 100A) for correcting for glass thickness is attached to the field lens; and a tongue provided on the objective side of the accommodator and projecting inward from an inner circumferential surface of the accommodator tube; at least two translucent plane parallel plates each having two mutually-parallel flat surfaces, wherein it is possible to insert one of the plane parallel plates into the accommodator and also to insert two combined plane parallel plates into the accommodator such that the respective two flat surfaces are orthogonal with respect to the optical axis of the field lens in a state where the adapter (100; 100A) for correcting for glass thickness is attached to the field lens; and a retainer attached to an end of the accommodator closest to the objective, the retainer holding the plane parallel plates between the retainer and the tongue in a state where the plane parallel plates are positioned within the accommodator, and wherein the accommodator has tubular length configured to accommodate at least the two plane parallel plates therein.