To provide an optical system which does not change in the intensity distribution of a laser light flux on a magneto-optical disk at the time of fine tracking by deflection and has little loss in a coupling efficiency to an object optical system, and is, moreover, not increased in the size of a coarse-moving arm.

This system is an optical system of an optical information recording and reproducing head wherein after a luminous flux emitted from a laser light source is made into a parallel luminous flux, it is made incident to an object optical system via a deflecting means and focused on an optical disk 2, and wherein at least one positive lens group and one negative lens group are combined between the deflecting means 26 and the object optical system and are constituted so that the vicinity of the center of rotation of the deflecting means 26 and a main plane position of the above-mentioned object optical system become approximately conjugate.