Disclosed is a method for the fabrication of a spot-size converter with a lateral-tapered waveguide (or an active layer), which utilizes a mask during a lithographic process wherein the mask has a pad that can absorb strain to be occurred during forming a lateral-tapered waveguide pattern at its distal end and the lateral-tapered waveguide is fabricated by forming the distal end on the order of about 0.6 &mgr;m in width followed by forming the lateral-tapered waveguide on the order of 0.1 &mgr;m using an wet etching. Thus, it is possible to reduce a fabrication cost because it is free from a high-resolution electron beam lithography and a stepper, and hence enhance a reproducibility of the lateral-tapered waveguide because it is free from an excessive wet etching during the use of a contact exposure equipment. Further, it is possible to integrate the spot-size converter fabricated by the above with an optical device, resulting in an increased position adjustment and reproducibility of the spot-size converter, which in turn, leads to increase in yield for the optical device.