The present invention provides a copolymerized polyester resin which can produce an adhesive resin having an excellent color tone and an excellent durability when used for an adhesive to be applied to various uses. A copolymerized polyester resin, comprising, as constituting ingredients, a dicarboxylic acid ingredient containing 75 molar % or less of terephthalic acid and a diol ingredient containing two or more kinds of diols, characterized in that the copolymerized polyester resin satisfies the following (1) and (2): (1) A decrease in a reduced viscosity after the copolymerized polyester resin is subjected to a heat treatment in a nitrogen atmosphere at 275° C. for 2 hours is 0.20 dl/g or less; and (2) A color b value is 5 or less.