PURPOSE: To obtain a reaction-injection-molded elastomer excellent in flowability and modulus by reacting a composition containing respectively specified polyether polyol, chain extender, polyisocyanate and organic catalyst in a closed mold.

CONSTITUTION: A composition containing a polyether polyol (e.g. polyoxyalkylene polyether triol derived from glycerol) having an average molecular weight of 500 or above, an amine-terminated chain extender (e.g. diethyltoluenediamine), an aromatic polyisocyanate (e.g. 4,4'-diphenyl diisocyanate) and an organotin catalyst represented by the formula [wherein R is a 1-8C alkyl or an aryl; Y is -CH2CH2-, -CH2CH2CH2-, -CH2-CH(CH3)-, -CH2CH(CH2OH)- or -CH2CH(OH)CH2-; and X is S or O] are reacted in an closed mold.