An optical waveguide multiplexer for use in optical fiber amplifiers or the like, which is simple in configuration, permits manufacturing at high productivity, and is reduced in size and moreover susceptible to little insertion loss, is to be realized. A multiplexer-branching filter is provided with first to fourth input/output (I/O) terminals. Part of an optical signal emitted from the first I/O terminal is reflected by a branching film, formed over one face of a glass plate, and led to the second optical I/O terminal. The rest of the optical signal penetrates the glass plate, and further penetrates an optical multiplexing film, formed on the other face, to be led to the third optical I/O terminal. The invention has a particular characteristic that the thickness t of the substrate is set so as to satisfy the condition of t.gtoreq.w/cos.theta..times.tan[sin.sup.-1 {(sin.theta.)/n}] where n is the refractive index of the substrate; w, the beam diameter of the output light of the optical I/O terminal for multiplexing the signal light with an pomping light; and .theta., the angle of incidence from this terminal to the substrate. By setting the substrate thickness at or above the value stated above, the optical path of any leak light from the pomping light side is deviated from that toward the monitor side, and no pomping light can enter into the monitor. It is also possible, by adding an aperture film to the substrate, to further reduce the leak light.