PURPOSE: To enable light to travel effectively by a method wherein a first and a second waveguide layer larger than a substrate in refractive index and an upper layer whose refractive index is smaller than that of the substrate are provided, and the thickness ratio of the first waveguide layer to the second waveguide layer is specified.

CONSTITUTION: A guide layer 11 formed of GaInAsP 3.2 in refractive index, 0.15μm in thickness, and 1.1μm in PL emission wavelength, a stop layer 12 formed of InP 200 in thickness, an active layer 14 formed of GaInAsP 3.5 in refractive index, 0.15μm in thickness (T1), and 1.55μm in PL emission wavelength, and an absorbing layer 15 formed of GaInAsP 3.43 in refractive index and 1.43μm in PL emission wavelength adjacent to the active layer 14 are formed on an InP substrate 10. Furthermore, the layers 14 and 15 are covered with an upper layer 16 of InP 3.098 in refractive index. At this point, the thickness t1 of the active layer 14 to the thickness t2 of the absorbing layer or the thickness ratio t2/t1 is 1:1/3-3. By this setup, light can be lessened in reflection at a joint between the waveguide layers of optical semiconductor elements, and in result light can be effectively transmitted.