A light collecting optical system wherein the position where light is collected is scanned in the optical axis direction by using a wavefront converting element capable of changing power without using a mechanical device, and aberration occurring during the scanning is canceled by using the wavefront converting element to minimize the degradation of light collecting performance due to the scanning in the optical axis direction. In a laser beam collecting optical system having an optical system for collimating a laser beam emitted from a laser light source and an objective for collecting the laser beam on a specimen, when the position where the laser beam is collected is changed in the optical axis direction in such a manner that the relative distance between the objective and the specimen does not change, a wavefront shape formed in an area covering at least 90% of the laser beam at or near the pupil position of the objective is changed by a wavefront converting element placed at or near a position conjugate to the objective pupil position so that predetermined conditions are satisfied.