Abstract: The present invention provides a thermosetting polyurethane elastomer that is yellowing-free, has low hardness and does not give rise to bleeding without using any plasticizer, has little tack, and exhibits small changes in hardness with temperature. This is achieved by a composition forming a yellowing-free, low-hardness polyurethane elastomer, containing no plasticizer and having (A) an isocyanate-terminated prepolymer and (B) a polyester polyol, wherein the isocyanate-terminated prepolymer (A) is an isocyanate-terminated prepolymer obtained by reacting hexamethylene diisocyanate with a glycol having an alkyl group as a side chain and having a molecular weight no greater than 500, and the polyester polyol (B) is a polyester polyol obtained from trimethylolpropane, 3-methyl-1,5-pentanediol and adipic acid, and having an average number of functional groups of 2.5 to 3.5 and a number average molecular weight of 800 to 5,000.