A copolymerized polyester resin containing a unit (A) represented by the following formula (1), a diol unit (B), and a unit (C) derived from a dicarboxylic acid or an ester-forming derivative of the dicarboxylic acid, wherein a content of the unit (A) based on total units of the copolymerized polyester resin is 10 to 95 mol %, and the copolymerized polyester resin satisfies the following conditions (1) to (3): (1) a glass transition temperature of the copolymerized polyester resin is 90° C. or higher; (2) an amount of heat generated by the copolymerized polyester resin at cooling crystallization is 5 J/g or less; and (3) an absolute value of photoelastic coefficient of the copolymerized polyester resin is 40×10−12 Pa−1 or less: wherein R1 is a hydrogen atom, CH3, or C2H5, R2 and R3 are each independently a hydrogen atom or CH3, and n is 0 or 1.