An optical transmitting device which combines a plurality of signal lights, each having a different, corresponding, frequency, into a wavelength division multiplexed signal light. The frequencies are spaced apart from each other in a manner with eliminates the effects of four-wave mixing (FWM). More specifically, the difference in frequencies of any pair-combination of the signal lights is different from the difference in frequencies between any other pair-combination of the signal lights. Moreover, the plurality of signal lights include n signal lights having respectively corresponding frequencies f1 through fn arranged in order from f1 to fn along a frequency spectrum, where frequencies f1 through fn−1 have respectively corresponding integer spacing coefficients m1 through mn−1, and frequencies fi and fi+1 are separated by mi·&Dgr;fS−F, where i=1 to (n−1) and &Dgr;fS−F is a unit of spacing between frequencies. m1 through mn−1 are all different from each other. In addition, m1 through mn−1 and the sum of any consecutive (n−2) spacing coefficients m1 through mn−1 should all be different from each other. The sum of m1 through mn−1 can be minimized to minimize required bandwidth.