An optical integrated circuit 1 according to the present invention includes a planar lightwave circuit 2, and a semiconductor element 3, which are fixed at one contact surface 12. A semiconductor optical amplifier (SOA) 9 is formed on a semiconductor substrate 8. A semiconductor waveguide 10 and a semiconductor waveguide 11 are formed on an input side and an output side of SOA 9, respectively. The semiconductor waveguide 11 has a turnaround portion 11a turned around on the semiconductor substrate 8. Respective ends of the optical waveguides 5 and 6 on a PLC platform 4 and respective ends of semiconductor waveguides 10 and 11 are optically coupled with each other at the contact surface 12.