A first clad layer is formed on a first substrate, and further first cores extending in parallel to each other in one direction are formed on the first clad layer. Separately from this, a second clad layer is formed on a second substrate, and further second cores extending in parallel to each other in a direction parallel to the first cores are formed on the second clad layer. Next, the first substrate and the second substrate are integrated with a third clad layer interposed therebetween in such a manner that a surface of the first substrate having the first cores formed thereon faces a surface of the second substrate having the second cores formed thereon, to thereby form an optical waveguide.