The FM noise spectrum and the spectral width of semiconductor lasers are measured in the high-power state up to 20 mW. The FM noise spectrum consists of the white noise and the 1/f noise. The spectral density of the white noise is suppressed by the increase in the output power, whereas that of the l/f noise is kept constant. This fact means that the residual linewidth in the high-power limit is caused by the l/f noise rather than the white noise.