An optical device that connects, by an optical signal, between an input signal and an output signal, the optical device comprising: an optical transmission line (101, 201) being sheet-form and having an incident surface (102, 202) for making an incident beam (107, 207) incident and an exit surface (103, 203) for making an exiting beam (108, 208) exit, wherein the optical transmission line (101, 201) includes a refractive index distribution such that a highest refractive index part is provided in a central position in a direction of a thickness of the optical transmission line (101, 201) and a refractive index decreases with distance from the highest refractive index part in the direction of the thickness, wherein the optical transmission line (101, 201) is capable of guiding waves in multiple modes having a plurality of eigenmodes excited in a direction of a width that is orthogonal to the direction of the thickness, wherein the optical transmission line (101, 201) transmits the incident beam (107, 207) in the multiple modes in the direction of a length that is orthogonal to the direction of the thickness and the direction of the width, and generates an exiting beam (108, 208) by changing an optical space distribution of the incident beam (107, 207) by mutual interference of the multiple modes, wherein the optical transmission line (101, 201) is formed from a polysilane in which oxygen or an oxide is previously diffused in a predetermined distribution, wherein the refractive index distribution is provided by an oxygen or oxide concentration distribution when the polysilane is cured, and wherein the refractive index distribution is provided by the ratio between the polysilane structure part that is not oxidized and the siloxane structure part that occurs due to oxidization at the time of curing. an optical transmission line (101, 201) being sheet-form and having an incident surface (102, 202) for making an incident beam (107, 207) incident and an exit surface (103, 203) for making an exiting beam (108, 208) exit, wherein the optical transmission line (101, 201) includes a refractive index distribution such that a highest refractive index part is provided in a central position in a direction of a thickness of the optical transmission line (101, 201) and a refractive index decreases with distance from the highest refractive index part in the direction of the thickness, wherein the optical transmission line (101, 201) is capable of guiding waves in multiple modes having a plurality of eigenmodes excited in a direction of a width that is orthogonal to the direction of the thickness, wherein the optical transmission line (101, 201) transmits the incident beam (107, 207) in the multiple modes in the direction of a length that is orthogonal to the direction of the thickness and the direction of the width, and generates an exiting beam (108, 208) by changing an optical space distribution of the incident beam (107, 207) by mutual interference of the multiple modes, wherein the optical transmission line (101, 201) is formed from a polysilane in which oxygen or an oxide is previously diffused in a predetermined distribution, wherein the refractive index distribution is provided by an oxygen or oxide concentration distribution when the polysilane is cured, and wherein the refractive index distribution is provided by the ratio between the polysilane structure part that is not oxidized and the siloxane structure part that occurs due to oxidization at the time of curing.