First, half etching is performed to a semiconductor layer formed on an insulating layer to form trenches at positions of slab-portion regions in which slab portions are to be formed. After filling the trenches with an insulating film, a resist mask which covers the semiconductor layer at a projecting-portion region in which a projecting portion is to be formed and whose pattern ends are located on upper surfaces of the insulating films is formed on upper surfaces of the semiconductor layer and the insulating film, and full etching is performed to the semiconductor layer with using the resist mask and the insulating film as an etching mask, thereby forming an optical waveguide constituted of the projecting portion and the slab portions. Thereafter, a first interlayer insulating film is formed to cover the optical waveguide.