A method of producing a curable composition containing a prepolymer having a polythioether skeleton, characterized by comprising addition reacting (A) 4,4'-bis(methacryloylthio)-diphenylsulfide having the formula (I): ##STR1## wherein R is a polyvalent organic group composed of an aliphatic or aromatic hydrocarbon and n represents an integer of 2 or more, with the functional group equivalent ratio of the mercapto groups of the component (B) (ii) to the methacryloyl groups of the component (A) being from 0.02 to 1.01, in another vinyl monomer copolymerizable with 4,4'-bis(methacryloylthio)diphenylsulfide having the above formula (I) in the presence of a base catalyst.