An optical multiplexer/demultiplexer, comprising: a substrate; an insulating layer; a semiconductor core layer; and rib type optical waveguides suitable for operation in the wavelength band of 1530 to 1610 nm, each rib type optical waveguide being formed on said substrate in said insulating layer and said semiconductor core layer; wherein the optical multiplexer/demultiplexer further comprises four of said rib type optical waveguides in the form of two single mode input optical waveguides (300a, 300b) and two single mode output optical waveguides (400a, 400b); four of said rib type optical waveguides in the form of reversible tapered regions (500a, 500b, 500c, 500d) including 2x2 waveguides that smoothly connect the input optical waveguides (300a, 300b) and the output optical waveguides (400a, 400b) to a further one of said rib type optical waveguides in the form of a multi-mode optical interference region; and the multi-mode optical interference region formed on an upper surface of the substrate, wherein each of both side surfaces of a core of the multi-mode optical interference region forms a stepped part (620a, 620b); and wherein a height of each stepped part (620a, 620b) is approximately equal to a half of a propagation wavelength in the wavelength band; and wherein the input optical waveguides, the output optical waveguides, the multi-mode optical interference region, and the reversible tapered regions are formed in the semiconductor core layer; and wherein a width and the height of the stepped parts (620a, 620b) are configured such that a first difference of the multi-mode optical interference region and a second difference of the multi-mode optical interference region are approximately equal to each other, the first difference being a difference between an effective refractive index of a fundamental mode and an effective refractive index of a first high order mode when a main electric field component in the multi-mode optical interference region is horizontal to the substrate, the second difference being a difference between an effective refractive index of the fundamental mode and an effective refractive index of the first high order mode when the main electric field component in the multi-mode optical interference region is perpendicular to the substrate; and whereinthe width of each stepped part (620a, 620b) is approximately equal to the propagation wavelength in the wavelength band. a substrate; an insulating layer; a semiconductor core layer; and rib type optical waveguides suitable for operation in the wavelength band of 1530 to 1610 nm, each rib type optical waveguide being formed on said substrate in said insulating layer and said semiconductor core layer; wherein the optical multiplexer/demultiplexer further comprises four of said rib type optical waveguides in the form of two single mode input optical waveguides (300a, 300b) and two single mode output optical waveguides (400a, 400b); four of said rib type optical waveguides in the form of reversible tapered regions (500a, 500b, 500c, 500d) including 2x2 waveguides that smoothly connect the input optical waveguides (300a, 300b) and the output optical waveguides (400a, 400b) to a further one of said rib type optical waveguides in the form of a multi-mode optical interference region; and the multi-mode optical interference region formed on an upper surface of the substrate, wherein each of both side surfaces of a core of the multi-mode optical interference region forms a stepped part (620a, 620b); and wherein a height of each stepped part (620a, 620b) is approximately equal to a half of a propagation wavelength in the wavelength band; and wherein the input optical waveguides, the output optical waveguides, the multi-mode optical interference region, and the reversible tapered regions are formed in the semiconductor core layer; and wherein a width and the height of the stepped parts (620a, 620b) are configured such that a first difference of the multi-mode optical interference region and a second difference of the multi-mode optical interference region are approximately equal to each other, the first difference being a difference between an effective refractive index of a fundamental mode and an effective refractive index of a first high order mode when a main electric field component in the multi-mode optical interference region is horizontal to the substrate, the second difference being a difference between an effective refractive index of the fundamental mode and an effective refractive index of the first high order mode when the main electric field component in the multi-mode optical interference region is perpendicular to the substrate; and whereinthe width of each stepped part (620a, 620b) is approximately equal to the propagation wavelength in the wavelength band.