An optical integrated circuit (1, 1') comprising: an SiO 2 buried oxide film layer stacked on a substrate in a laminating manner, an Si core layer stacked on the SiO 2 buried oxide film layer in a laminating manner, and a top clad layer stacked on the Si core layer in a laminating manner and having an index of refraction lower than that of Si; an optical circuit configured to generate signal light having a wavelength "λ"; a spot size converter configured to receive the signal light from the optical circuit, the optical circuit being configured to provide the signal light with a spot size "w" on a coupling end-surface of the spot size converter; a multimode optical waveguide element (40, 40'); and an optical waveguide having a curved part, the optical waveguide being configured to guide the signal light from the spot size converter to a light-receiving surface of the multimode optical waveguide element (40, 40'), wherein, on a plane of the optical integrated circuit (1, 1'), a position (x s, y s) of a point on the end-surface of the spot size converter and a position (x m, y m) of a point on the light-receiving surface of the multimode optical waveguide element (40, 40') satisfy a relationship x m − x s > y m − y s tan θ, wherein θ = arcsin λ / λ 2 + π 2 n 2 w 2 / 4 1 / 2, and wherein in the case that an index of refraction of light of the SiO 2 buried oxide film layer is represented by "n box," an index of refraction of light of the top clad layer is represented by "n clad," a thickness of the SiO 2 buried oxide film layer is represented by "T box," and a thickness of the top clad layer is represented by "T clad," n is determined as n ≈ n box T box + n clad T clad / T box + T clad, the optical integrated circuit (1, 1') is configured such that, in use, a portion of the signal light received by the coupling end-surface of the spot size converter is repeatedly reflected by a bottom surface of the SiO 2 buried oxide film layer and a top surface of the top clad layer and propagates therebetween as stray light, the stray light having a distribution of stray light intensity on the plane of the optical integrated circuit (1, 1'), and the optical waveguide extends linearly from the spot size converter and in a waveguiding direction of the signal light at the spot size converter, and starts curving at a curving starting position of the curved part and continues curving for approximately a quarter of a circumference, the curving starting position being located on the plane of the optical integrated circuit (1, 1') such that an optical loss due to interference between the stray light and a remaining portion of the signal light received by the coupling end-surface of the spot size converter occurring during use of the optical integrated circuit (1, 1') is minimized. an SiO 2 buried oxide film layer stacked on a substrate in a laminating manner, an Si core layer stacked on the SiO 2 buried oxide film layer in a laminating manner, and a top clad layer stacked on the Si core layer in a laminating manner and having an index of refraction lower than that of Si; an optical circuit configured to generate signal light having a wavelength "λ"; a spot size converter configured to receive the signal light from the optical circuit, the optical circuit being configured to provide the signal light with a spot size "w" on a coupling end-surface of the spot size converter; a multimode optical waveguide element (40, 40'); and an optical waveguide having a curved part, the optical waveguide being configured to guide the signal light from the spot size converter to a light-receiving surface of the multimode optical waveguide element (40, 40'), wherein, on a plane of the optical integrated circuit (1, 1'), a position (x s, y s) of a point on the end-surface of the spot size converter and a position (x m, y m) of a point on the light-receiving surface of the multimode optical waveguide element (40, 40') satisfy a relationship x m − x s > y m − y s tan θ, wherein θ = arcsin λ / λ 2 + π 2 n 2 w 2 / 4 1 / 2, and wherein in the case that an index of refraction of light of the SiO 2 buried oxide film layer is represented by "n box," an index of refraction of light of the top clad layer is represented by "n clad," a thickness of the SiO 2 buried oxide film layer is represented by "T box," and a thickness of the top clad layer is represented by "T clad," n is determined as n ≈ n box T box + n clad T clad / T box + T clad, the optical integrated circuit (1, 1') is configured such that, in use, a portion of the signal light received by the coupling end-surface of the spot size converter is repeatedly reflected by a bottom surface of the SiO 2 buried oxide film layer and a top surface of the top clad layer and propagates therebetween as stray light, the stray light having a distribution of stray light intensity on the plane of the optical integrated circuit (1, 1'), and the optical waveguide extends linearly from the spot size converter and in a waveguiding direction of the signal light at the spot size converter, and starts curving at a curving starting position of the curved part and continues curving for approximately a quarter of a circumference, the curving starting position being located on the plane of the optical integrated circuit (1, 1') such that an optical loss due to interference between the stray light and a remaining portion of the signal light received by the coupling end-surface of the spot size converter occurring during use of the optical integrated circuit (1, 1') is minimized.