The Rayleigh scattering of the mixed-alkali glass system K2O-Na2O-MgO-SiO2 (KNMS) was investigated, both experimentally and theoretically. The lowest Rayleigh scattering coefficient (38% of that for pure SiO2 glass) was obtained when the glass composition was 22K(2)O-8Na(2)O-10MgO-60SiO(2) tin mol%). These values are equal to or less than the minimum values reported for the ternary sodium silicate glass Na2O-MgO-SiO2. The Rayleigh scattering caused by concentration fluctuation was believed to have been reduced greatly in this KNMS glass, because the mobility of the alkali-metal ions was reduced by the mixed-alkali effect.