Provided is a polyester resin having good film formability together with resisting film breakage (hair) and scraping (galling) in the drawing and ironing steps when applied to a polyester film for covering metal plates, and, as a film for the inner surfaces of metal cans, resisting corrosion of metal cans without damage to the film surface even if dropped or subjected to external shock (dented) after a product has been made by sealing contents inside a can. A copolyester resin characterized in including, as constituent units: 50-93 mass% of an ester oligomer (A) having a number-average molecular weight of 700 or lower, the ester oligomer (A) comprising a dicarboxylic acid unit (a1) containing 70 mol% or more of a terephthalic acid component and a diol unit (a2) containing 70 mol% or more of an ethylene glycol component; and 7-50 mass% of a polyester polyol (B) having a number-average molecular weight of 1500-3000, the polyester polyol (B) comprising a hydrogenated dimer acid unit (b1) and a 1,4-butanediol unit (b2).