A core and a slab layer that are formed on a lower clad layer are provided. The lower clad layer is formed on a substrate. The core is comprised of a semiconductor and has a rectangular shape in a cross-sectional view. The slab layer is comprised of a semiconductor. The core and the slab layer have a thickness that allows only up to a secondary mode of light to be present. Further, the core and the slab layer are laminated on the lower clad layer. Further, the core and the slab layer are disposed to be optically coupled to each other.