A statistical numerical simulation method is proposed, and by which the accumulation of phase fluctuation and the spectral linewidth in semiconductor lasers are investigated theoretically. In the case of single mode, the result is quite in agreement with that by small signal approximation instead of being its 4 times. In the case of nearly single mode, the linewidth of the main mode is increased somewhat but not too seriously by the side mode. The overestimation and its cause of the results obtained by a Monte Carlo method used by other author recently is noted, and a power independent linewidth component of the side mode of the order of 100 MHz agreed with experiment is obtained where the small signal approximation is not applicable.