PURPOSE: To provide a new polyester diol useful as a material for elastomer, paint, adhesive, elastic fiber, sealant, etc., required to have bio-degradability, exhibiting liquid state in spite of high molecular weight and having excellent bio-degradability, hydrophilic nature and compatibility.

CONSTITUTION: A polyester diol derived from (A) polyoxyethylene glycolic acid of formula (n is integer of 2-70), (B) one or more kinds of polyalkylene glycols and, as necessary, (C) a hydroxy alkylene carboxylic acid or its lactone. The compound can be produced by the esterification reaction of the components A and B. Since the compound has oxyethylene group on the main chain, it has excellent hydrophilic nature and compatibility to isocyanate. The polymer is polymerized through ester bond and, accordingly, it has excellent bio- degradability and exhibits liquid state in spite of high molecular weight. Accordingly, it is useful as a raw material for various industrial products mentioned above to be applied to polyurethane, polyester, epoxy resin, etc.