PURPOSE: To obtain an optical branching unit having uniform characteristics by inserting transparent bodies having a high refractive index and transparent bodies having a low refractive index into a cylindrical body, expanding the cylindrical body in the longitudinal direction to form the base material of the optical branching filter and then cutting off the base material vertically.

CONSTITUTION: One sides 3A of three high refractive-index transparent bodies in the longitudinal direction are butted against each other and combined radially an the radially combined unit is arranged in the cylindrical body 6. Then, plural rod-like low refractive-index transparent bodies 4 are inserted into respective gaps between respective transparent bodies 3 and the cylindrical body 6 and respective transparent bodies 3, 4 are arranged in the cylindrical body 6. Only the terminals are temporally fixed, and while being heated in a furnace, the transparent bodies 3 and 4 are united to remove the gaps. The cylindrical body 6 is heated moreover, the cylinder 6 and the transparent bodies 3 are softened, a tensile force is applied to the leading end of the cylinder 6 and the transparent bodies 3 to expand and unite them, so that the base material of the branching filter can be produced. The base material 8 is cut off with a prescribed thickness vertically to the longitudinal direction to form a disc body. Discoid transparent plates 5 are adhered to both the sides of the cut-off disc body and an optical path is formed by arranging the low refractive-index transparent bodies around the transparent bodies 3 to form the optical branching unit.