The invention provides a wavelength multiplexing transmission apparatus includes a plurality of optical signal outputting sections for outputting optical signals having different wavelengths from each other, and a wavelength multiplexing section for wavelength multiplexing the optical signals outputted from the optical signal outputting sections and sending out a resulting optical signal. Each of the optical signal outputting sections includes a transmission light source driven by an electric signal for outputting an optical signal of a predetermined wavelength, and a wavelength filter capable of passing therethrough and sending out only an optical signal of the predetermined wavelength to be outputted from the transmission light source to prevent a wavelength drift of the optical signal outputted from the transmission light source. A variation of the wavelength of the transmission light source is detected, and the transmission light source is shut down in order to prevent otherwise possible crosstalk with another channel and prevent a further influence of the same.