There is disclosed a waveguide-type wavelength multiplexing optical transmitter/receiver module which is composed of an optical demultiplexer having an optical waveguide substrate and a filter disposed on an optical waveguide substrate, a laser diode and a photodiode. On the optical waveguide substrate, a first optical waveguide and a second waveguide are formed. On a first end face of the optical waveguide substrate, there is a cross section in which these optical waveguides cross each other. On a second end face of the optical waveguide substrate, there is an optical fiber and ports for connecting the laser diode. The filter reflects light with a wavelength of &lgr;1 from the laser diode and allows light with a wavelength of &lgr;2 from the optical fiber to pass therethrough. The laser diode is so disposed that the location of a center axis is shifted in the direction of the first optical waveguide relative to a cross axis being normal to a first end face. The photodiode does not receive an unwanted light with a wavelength of &lgr;2 or so from the laser diode. The waveguide-type wavelength multiplexing optical trans-mitter/receiver module having excellent light receiving characteristics is realized.