To eliminate the need for a high-temp. reflow treatment and to avert the generation of a stress by a heat treatment by obviating the formation of voids between cores in forming an upper clad layer even if the plural cores are formed apart narrow spaced intervals.

A lower layer clad 2 consisting of a quartz material is formed by a CVD method on a substrate 1. A core forming material layer consisting of the quartz material is then formed by the CVD method on this lower clad. Further, at least two pieces of the channeled cores 3 are formed by patterning the core forming material layer. The upper layer clad 4 which consists of the quartz material contg. one or plural kinds of phosphorus, boron and Ge as a dopant and coats these cores is formed by the CVD method using an org. material as a source. The respective materials which are the sources have the same alkoxyl groups.