To obtain a copolyester resin for an adhesive improved in wet heat resistance while keeping good adhesion to a polyester film, and an adhesive using the same.

The copolyester resin for an adhesive has a glass transition temperature of at most 30°C and a reduction rate of a number-average molecular weight caused by wet heat treatment at a temperature of 85°C, at a humidity of 95% and for a period of 500 h is less than 35%.