PURPOSE: To enable easy correction of an aberration even if a distributed index lens is used by shielding partly the luminous flux passing through the peripheral part of the lens.

CONSTITUTION: The refractive index n(r) of a distributed index lens which is good in aberration in the position apart by (r) from the optical axis of said lens is expressed by the equation with the axial refractive index as n0 in which (g) is the value to determine the power of inhomogeneous paraxial quantity, (hi) is the value relating to the (i-1)th aberration and both are constants. A lens 12 is the distributed index lens manufactured by controlling the term up to h6 of said equation in the stage of manufacture. The luminous flux from a semiconductor laser 11 is collimated by the lens 12 and is emitted therefrom. A stop 13 which limits the diameter of the luminous flux is provided on the exit plane and the diameter D2 of the stop is smaller than the diameter D1 of the lens 12. The ghost light generated on the outside circumference of the lens 2 is shut off and the aberration is maintained satisfactorily by the simple method.