PURPOSE: To enable the observation of a good image having a high contrast by arranging a quarter-wave plate in such a manner that the normal at its surface inclines with an optical axis formed by an objective lens and an imaging lens.

CONSTITUTION: This cofocusing optical microscope has the quarter-wave plate 72 arranged on the optical path between the first imaging lens 51 and the first objective lens 41 and an optical axis correction plate 75 arranged on the optical path between the quarter-wave plate 72 and the first objective lens 41. The quarter-wave plate 72 is so arranged that the normal at its surface inclines with the optical axis (reference optical axis) formed by the first objective lens 41 and the first imaging lens 51. The quarter-wave plate 72 is arranged to incline with the reference optical axis n such a manner, by which the reflected light is not condensed to the pinhole in a region A to be irradiated with a spot even if a part of the light from the pinhole of a rotary scanning disk 21 is reflected by the front and rear surfaces of the quarter-wave plate 72.