An efficient method is proposed for the determination of the concentration of nitrogen in an indirect-transition compound semiconductor such as gallium phosphide added as an isoelectronic trap. The method utilizes the fact that a good correlation of proportionality is held between the nitrogen concentration and the difference .DELTA..alpha.(=.alpha..sub.N -.alpha.) in the absorption coefficient of light of a wavelength identical with the wavelength .lambda..sub.N due to the excitons under constraint in the isoelectronic trap between the semiconductors with (.alpha..sub.N) and without (.alpha.) addition of nitrogen. A working curve is presented between the nitrogen concentration determined by the method of SIMS and the value of .DELTA..alpha..