PURPOSE: To reduce a parasitic capacity and a dark current and realize a high- speed operation of an optical integrated element by a method wherein a step produced between a semiconductor laser and a photodiode and a field effect transistor is filled with SiO2.

CONSTITUTION: A groove with a reverse trapezoid cross-section is formed in a semi-insulating GaAs substrate 1. An N+ type layer is deposited over the whole surface by liquid phase deposition and the N+ layer 3 is removed by etching except the part on the bottom surface in the groove. Then an I-type GaAs layer, an I-type GaAlAs window layer and an I-type GaAs cap layer are deposited by liquid phase deposition over the whole surface and etched. After SOG is applied by spin coating, a step in the semi-insulating GaAs substrate is filled by baking. Then, after the implantation of an N+ type layer d14 and the implantation of an N-type layer 13 are carried out by utilizing a resist 16 as a mask, positive side diffusion of a photodiode is carried out. Further, a source electrode 10, a drain electrode 11 and a negative side electrode 12 of the photodiode are deposited by a vacuum evaporation and a positive side electrode 18 and a gate electrode 9 are formed.