A recording reproducing optical system for an optical information recording medium is provided with a light source; an objective lens; and an aberration correcting optical element provided on an optical path between the light source and the objective lens. The objective lens includes an aspherical surface and one diffractive surface on which a diffractive structure structured by a plurality of concentric ring-shaped zonal steps is formed and the objective lens is a plastic lens structured in one element in one group. The aberration correcting optical element comprises at least one plastic lens on which a diffractive structure structured by a plurality of concentric ring-shaped zonal steps is formed. The following formulas are satisfied: PD1>0 (PD1 is a diffractive power (mm−1)), PD2 0 (PR2 is a refractive power (mm−1) as a refractive lens).