To provide an image forming optical system of high performance with a small number of constitutional lenses, capable of satisfactorily dealing with many continuous changes of the disk substrate thickness at recording/ reproducing a high density and mass storage optical information medium, irrespective of the extremely simple constitution of the system.

A 1st positive lens, a 2nd negative lens, an objective lens and the disk substrate are successively arranged with reference to a parallel luminous flux from a collimator, and by varying a distance on the optical axis between the 1st positive lens and the 2nd negative lens against the increase of the aberration caused by the change of the disk substrate thickness, the aberration is reduced, and by slightly moving the objective lens on the optical axis against the shift of the image point position caused by the change of the disk substrate thickness, the focusing is accomplished.