As a fundamental study for the application of 5'-D-riboflavin-α-D-glucopyranoside (B_2-5'-α-gl) as food additive, both the influence of light on B_2-5'-α-gl and its degree of bitter taste were examined. B_2-5'-α-gl dissolved in buffer solution of pH 3,6 and 8 was found always to be more stable for light than riboflavin and flavin mononucleotide. It was pointed out from organoleptic examination that a bitter taste of riboflavin was fairly stronger than that of B_2-5'-α-gl.