Abstract: The focal point detection method detects focal point dislocation of a coupling objective lens in accordance with a light beam passing a 60% to 80% region of a light beam effective diameter, which is centered with respect to an optical axis OZ of a light beam passing through the coupling objective lens, and which is regulated by a numerical aperture of the coupling objective lens. In this manner, focal point dislocation of a converging optical system can be performed without an offset, thereby accurately forming a focal point of the converging optical system on an information recording layer of an optical recording medium.

Abstract: A combinational object lens 3 constructed of a first lens 1 and a second lens 2 is formed. The first lens 1 and a first lens barrel 4 for retaining the first lens 1 are fixed by bonding to a second lens barrel 5 for retaining the second lens 2 only via a spacer layer 7 made of an adhesive. With this arrangement, residual aberration of the combinational object lens constructed of a plurality of lenses can be reduced, and this further enables the compacting and reduction in weight of the object lens barrels, an object lens barrel drive unit and an optical information recording and reproducing unit.