An integrated optical device comprising a first semiconductor optical element provided on a first region of the main face of a substrate and a second semiconductor optical element provided on a second region and optically coupled to the first semiconductor optical element is fabricated. A first III-V compound semiconductor layer containing Al element is formed on the main face. A second III-V compound semiconductor layer for forming the first semiconductor optical element is then formed on the first III-V compound semiconductor layer. An etching mask M is formed on the first region. The end point of the dry etching is detected by using the etching mask M to dry-etch the second III-V compound semiconductor layer while detecting Al element. The first semiconductor optical element is thus formed. The second semiconductor optical element is formed on the second region.