PURPOSE: To adjust the distance between body images over a wide range by varying the remaining refracting index of one lens in the direction of an optical axis except a part which has specific thickness in the optical-axis direction and also varying the refractive index of the other lens radially from the optical axis to the outer periphery.

CONSTITUTION: Divergent light beams emitted by a light source 1 are made incident on an objective 10 and converged on the recording surface 5A through the optical lens 10. Then, the lens 10 is constituted by arranging the convex surfaces of two plano-convex lenses 2 and 4 opposite each other in contact and fixing them in a common lens barrel 6. This lens barrel 6 is moved by an external driving mechanism to and away from the surface of the disk 5 to make a focus adjustment. Then, the lens 2 is constant in refractive index at the constant-distance thickness pare from the lens surface 2A on the light source side in the optical axis direction and varies in refractive index at the remaining part in the optical-axis direction and also has a constant refractive index distribution in a plane perpendicular to the optical axis. The object-side lens 4 varies in refractive index gradually and radially from the optical axis to the outer periphery and has a constant refractive index distribution in parallel to the optical axis.