PURPOSE: To provide the optical system which is improved to have no spherical aberration and couples a laser, specially, a semiconductor laser with an external optical element.

CONSTITUTION: The optical system 100 comprises of the semiconductor laser 1 having a front faucet R1 and a rear faucet, a lens 3, and an adhesive material 2 which adheres the lens 3 and the front faucet R1 of the laser 1 together. When the laser 1 is coupled with the external optical element, the piano-convex lens 3 is fitted to the front faucet R1 of the laser 1 by applying the adhesive material 2 which has the same refractive index with the lens 3. The combination of the lens 3 and adhesive material 2 form an optical system 100 having no spherical aberration and the numerical aperture of the optical system 100 is improved.