An optical recording/reproducing apparatus is arranged so that, upon recording
or reproducing a recording medium having a plurality of recording layers, the gap
of the lens groups at the time of forming a converged light spot on this first
recording layer is DIS(1), the gap of the lens groups at the time of forming
the converged light spot on the N-th layer (the farthest layer from the surface
of the recording medium) is DIS(N) and the neutral point of the spherical-aberration
correcting mechanism is set at a position satisfying the following expression:
lens group gap dst(3)=((DIS (1)+DIS(N))/2.
