PURPOSE: To obtain an optical system having sufficient performance even against temperature variation and variation in the distance between object images while maintaining necessary image formation power and the distance between the object images although the NA is large and the absolute value of lateral power is large as the optical system for the recording and reproduction of the optical information medium.

CONSTITUTION: The optical system consists of a coupling lens 3 as a single lens having at least one aspherical surface and an objective 2 which has aspherical surfaces on both the light source side and image side in order from the light source side; and the light source 4 is arranged closer to the coupling lens than the light source side focus of the coupling lens and the objective is movable and satisfies -1/10 ≤ mo ≤-1/20 and -112 ≤ mt ≤ -11/6, where mo is the lateral power of the objective itself and mt is the lateral power of the optical system.