An optical system of an optical information recording/reproducing apparatus includes a light source for emitting a generally parallel luminous flux. An objective optical system for converging the luminous flux emitted from the light source portion onto a medium is provided. The objective optical system can comprise a hologram lens or can have both surfaces formed as convex aspherical surfaces. A chromatic aberration correcting element having almost no power is adapted to correct chromatic aberration of the objective lens. A mechanism for independently actuating the objective lens at least in an optical axis direction is provided. Further, a beam splitter for splitting the luminous flux reflected by the medium from a light path directed to the light source portion and guiding the luminous flux to a light receiving system can be provided.