Provided are an optical transmission apparatus and method using a light source for wavelength division multiplexing (WDM) optical communication that employs a Fabry-Perot laser diode (F-P LD) whose output wavelength is locked by an externally injected incoherent light, a multifiber, and a waveguide grating router. The light transmission apparatus includes: an incoherent light source (ILS) outputting incoherent light; a plurality of circulators (CIRs) connected to the ILS, receiving the incoherent light from the ILS, and outputting first optical signals; a first waveguide grating router (WGR) outputting the first optical signals output from each of the CIRs to optical fibers corresponding to each of the CIRs, and outputting second optical signals input from the optical fibers to the corresponding CIRs; a plurality of second WGRs corresponding to each of the CIRs, and demodulating the second optical signals output from each of the plurality of CIRs; and a plurality of receivers connected to the plurality of the second WGRs, and inputting the demultiplexed optical signals output from the plurality of second WGRs. A plurality of light sources for WDM optical communication whose output wavelength is locked can increase size and economical efficiency of a light transmission system (subscriber). The N.times.N WGR can produce a conventional light transmission system and accommodate many subscribers.