PURPOSE: To obtain a polyurethane elastomer, having fine cells, a relatively low expansion ratio, a low compression set and a microcellular structure and excellent in mechanical properties such as tensile strength or tear strength.

CONSTITUTION: This polyurethane elastomer, having a microcellular structure is obtained by reacting a polyether polyol with MDI at 1:0.65 weight ratio, then providing a prepolymer with 8% terminal isocyanate content, on the other hand, preparing a polyol component comprising a trifunctional polyether polyol having 6000 number-average molecular weight, water and 1,4-butanediol at 1:1.6 molar ratio, a catalyst and a foam stabilizer, regulating the prepolymer and polyol component to respective prescribed temperatures, then mixing both at a prescribed ratio, stirring the mixture and subsequently casting the stirred mixture into a metallic mold.