A novel saccharide sensory system showing remarkable color changes has been developed. A copolymer containing boronic acid and cationic units was synthesized on which anionic dyes were adsorbed. By immersing the dye-adsorbed copolymer in aqueous saccharide solutions, blue and yellow dyes were sequentially released from the copolymer: with increasing saccharide concentration, the solution changed its color from colorless to blue, then changed into green.