The optical multiplexer/demultiplexer 100 includes a member 110 on which optical waveguides 111 to 114 and a groove 115 are formed and an optical filter 120 inserted in the groove 115. The member 110 is a planar optical waveguide where the optical waveguides 111 to 114 are formed on one surface thereof, and the groove 115 is formed thereon. Before the groove 115 is formed, the optical wave guide 111 and the optical waveguide 114 are a continuous optical waveguide; and the optical waveguide 112 and the optical waveguide 113 are a continuous optical waveguide. A straight groove 115 is formed so as to go through an intersecting point of the two continuous optical waveguides. The optical waveguides 111 and 114 are formed straightly on the member 110.