An optical wave guide has a lower cladding layer formed on a substrate, and a Y-branched optical wave guide arranged on the lower cladding layer. The optical wave guide is covered with an upper cladding layer. Curved portions of the optical wave guide are tapered at the external circumferential sides and formed into a trapezoidal cross sectional shape, and effective refractive index of the optical wave guide is made small at the tapered external circumferential sides. Meanwhile, end surfaces of the optical wave guide are formed into a rectangular shape, thereby joint efficiency with optical fibers is increased.