A non-reciprocal optical transmission device comprising: an optical input portion; an optical output portion; and an intermediate connecting portion interposed between the optical input portion and the optical output portion, and comprising at least first and second optical waveguides (W10, W20) optically connecting the optical input and output portions, and wherein the non-reciprocal optical transmission device further comprises a side patch disposed on a side of the intermediate connecting portion and to a side of the first optical waveguide, wherein either one or both of a width and a thickness of any one or any combination of the optical waveguides changes gradually in a direction (x) from the optical input portion to the optical output portion, and a distance between the side patch and the first optical waveguide changes gradually in a direction from the optical input portion to the optical output portion, thereby gradually changing a complex refractive index (n 1, n 2) of the first optical waveguide along the direction (x) from the optical input portion to the optical output portion; wherein a real part (n 1 ', n 2 ') and an imaginary part (n 1 ", n 2 ") of the complex refractive index (n 1, n 2) of any one or any combination of the optical waveguides changes gradually with the distance (x) along the waveguide between the optical input portion and the optical output portion, and a transmission direction of light through the non-reciprocal optical transmission device is controlled by a change in the complex refractive index; and wherein a change profile (Δn 1 (x), Δn 2 (x)) of at least one of the real part and the imaginary part of the complex refractive index in the direction from the optical input portion to the optical output portion and the change profile of the complex refractive index in the direction from the optical output portion to the optical input portion is asymmetric with respect to a center of the intermediate connecting portion. an optical input portion; an optical output portion; and an intermediate connecting portion interposed between the optical input portion and the optical output portion, and comprising at least first and second optical waveguides (W10, W20) optically connecting the optical input and output portions, and wherein the non-reciprocal optical transmission device further comprises a side patch disposed on a side of the intermediate connecting portion and to a side of the first optical waveguide, wherein either one or both of a width and a thickness of any one or any combination of the optical waveguides changes gradually in a direction (x) from the optical input portion to the optical output portion, and a distance between the side patch and the first optical waveguide changes gradually in a direction from the optical input portion to the optical output portion, thereby gradually changing a complex refractive index (n 1, n 2) of the first optical waveguide along the direction (x) from the optical input portion to the optical output portion; wherein a real part (n 1 ', n 2 ') and an imaginary part (n 1 ", n 2 ") of the complex refractive index (n 1, n 2) of any one or any combination of the optical waveguides changes gradually with the distance (x) along the waveguide between the optical input portion and the optical output portion, and a transmission direction of light through the non-reciprocal optical transmission device is controlled by a change in the complex refractive index; and wherein a change profile (Δn 1 (x), Δn 2 (x)) of at least one of the real part and the imaginary part of the complex refractive index in the direction from the optical input portion to the optical output portion and the change profile of the complex refractive index in the direction from the optical output portion to the optical input portion is asymmetric with respect to a center of the intermediate connecting portion.