An integrated
semiconductor optical device includes first and second
semiconductor optical devices. The first
semiconductor optical device includes a first core layer, a first upper cladding layer including a first
ridge portion, a first buried layer surrounding the first
ridge portion, and a first adjusting layer provided between the first buried layer and the first
ridge portion. The second semiconductor optical device includes a second core layer, a second upper cladding layer including a second ridge portion. The first semiconductor optical device and the second semiconductor optical device are arranged next to each other in a predetermined axis direction. The first core layer is joined to the second core layer by a
butt joint method at a joint boundary between the first and second semiconductor optical devices. The first adjusting layer has a
refractive index lower than a
refractive index of the first core layer and higher than a
refractive index of the first buried layer. The first adjusting layer extends in the predetermined axis direction. The first adjusting layer has a constant width from one end
facet to the joint boundary.