PURPOSE: To obtain the titled composition which is excellent in moldability and can give a rubber product, especially, a cast tire, excellent in mechanical properties and flex cracking resistance, by mixing a specified polyol compound with a polyisocyanate compound and a synthetic fiber.

CONSTITUTION: A polyol compound (A) having 0.1W10meq/g of internal or terminal OH groups, which is a liquid diene polymer of a number-average MW of 300W25,000, is mixed with a polyisocyanate (B) having at least two isocyanate groups reactive with the OH groups of component A in the molecule in an amount to provide a molar ratio of the NCO groups of component B to the OH groups of component A (NCO/OH) of 0.2W25 and 5W100pts.wt., per 100pts. wt. component A, synthetic fiber (C) (e.g., polyamide fiber) which is a short fiber of a length of 0.01W10cm, preferably, 0.05W5cm and has a fiber diameter of 1W5,000 denier.