In an MMI device with which a plurality of narrow width single-mode waveguides are provided at both ends of a multimode waveguide M, and light introduced into one single-mode waveguide on the input side is interfered in the multimode waveguide to be emitted from two single-mode waveguides on the output side, a wall face perpendicular to the optical axis direction that is not provided with a single-mode waveguide is formed with an inclination being given in the thickness direction, at both ends of the multimode waveguide. Thereby, the light which is incident on the wall face is reflected toward the substrate side. In this case, because the angle of incidence into the substrate is small, almost all of the light is radiated into the inside of the substrate without being returned to the single-mode waveguide on the incident side. Therefore, an MMI device which is capable of completely eliminating the return of the reflected light with a simple structure, and a manufacturing method for the same are provided.