Abstract: An OCT system includes an OCT optical system and a reference attachment. The OCT optical system has a beam splitter for splitting light into a measurement optical path and a reference optical path, a photodetector, a first waveguide, a second waveguide, and first connectors provided at end portions of the first waveguide and the second waveguide respectively for indirectly or directly connecting the first waveguide and the second waveguide. The reference attachment has a third waveguide, and second connectors formed at both ends of the third wave guide and configured to be attachable to and detachable from the first connector, and is attached to and detached from the OCT optical system h attaching and detaching each of the second connectors to and from the first connector to change an optical path length of the reference optical path.