An analytical method for the determination of sodium alginate in various foods was developed. Pectin and protein were removed from foods with 1.5mol/L sulfuric acid and 1mol/L magnesium sulfate-2.2mol/L sodium sulfate solution. Pectic substances in foods were removed as a precipitate with 1% sodium hydrogen carbonate solution. Alginic acid in the extract was precipitated as a copper salt, dissolved in 1mol/L ammonia water, and determined colorimetrically by means of a naphthoresorcinol reaction. The described method was applied to 18 kinds of food. The recovery of sodium alginate spiked at the level of 0.1% was more than 75%, except for soybean flour. The determination limit of the method was 0.002%. Sodium alginate was not detected in 18 kinds of food, but was detected in whipped cream, pudding and noodles at levels of 0.280%, 0.112% and 0.290%, respectively.