A method for fabricating low-loss optically active device having an optical waveguide constructed of an optical waveguide core region (non-linear core region) necessitating the non-linear effect when waveguiding an optical signal, and an optical waveguide core region (linear core region) not necessitating the non-linear effect, the method includes method for fabricating an optically active device having an optical waveguide constructed of an optical waveguide core region (non-linear core region) necessitating the non-linear effect when waveguiding an optical signal, and an optical waveguide core region (linear core region) not necessitating the non-linear effect, the method includes the steps of: forming a lower clad layer having a refractive index lower than the material of the waveguide core regions and optical transparency on a substrate, forming a linear optical polymer layer on the lower clad layer by coating linear optical polymer having a refractive index lower than the material of the lower clad layer, forming a first metal layer at a region on the lower clad layer, other than the regions where the waveguide is to be disposed, etching a linear optical polymer layer without the first metal layer formed thereon, forming a non-linear optical polymer layer on the substrate having the non-linear core region, removing the non-linear optical polymer layer stacked to be higher than the waveguide core regions, removing the first metal layer, forming a second metal layer on the waveguide from which the first metal layer is removed, removing the linear optical polymer of the non-second metal layer portion, and forming an upper clad layer on the substrate with the linear optical polymer using a material having a refractive index lower than the waveguide core regions and optical transparency. The waveguide is formed using non-linear optical polymer only at the region where the non-linear effect such as optical modulation or optical switching occurs, and is formed using linear optical polymer at the remaining regions, thereby minimizing the overall waveguiding loss of the waveguide.