An optical device comprising a substrate, a porous layer laid on the substrate having a pore diameter smaller than the wavelength of light and a crystal layer laid on the porous layer showing a refractive index greater than that of the porous layer is presented. The optical device is manufactured by a method comprising a step of forming a porous layer having a pore diameter smaller than the wavelength of light on the surface of a substrate and a step of forming a crystal layer showing a refractive index greater than that of the porous layer on the porous layer. Since the porous layer is clad, light can be confined with ease.