Abstract: An optical attachment for a microscope which comprises a relay lens, a field lens, and at least four mirrors forming a square, closed optical path. The relay lens is set between the third mirror and the fourth mirror, the field lens between the first mirror and the second mirror, while the field lens is set on the image erected by the relay lens, the objective of the microscope is so set as to focus on the image erected by the field lens, and the first mirror is made replaceable outside of an optical path formed in front of the objective of the microscope.