This is an optical apparatus provided with an objective with a correction collar for correcting aberration due to an error in the optical thickness of a piece of cover glass comprising a focusing mechanism for changing a distance between the objective and the sample, an optical thickness detecting unit for detecting the optical thickness of the cover glass, an operating unit for calculating the amount of aberration correction, based on the optical thickness of the cover glass detected by the optical thickness detecting unit, a driver unit for driving a correction collar, based on the amount of aberration correction calculated by the operating unit and an imaging sensor for forming the image of the sample that passes through the objective.