PURPOSE: To provide the titled composition composed of a specific liquid thioether-type polymerizable monomer and a polymerizable polyfunctional vinyl monomer, having excellent polymerization curability at normal temperature and giving a cured product having excellent hardness and strength.

CONSTITUTION: The objective composition is produced by compounding (A) ≥50wt.% of a liquid thioether-type monomer produced by the addition reaction of (A1) a polyene compound of formula I [R1 and R2 are H or CH3; R3 is 2W20C (OH-containing) aliphatic polyhydric alcohol residue or polyalkylene ether polyol residue; m is 1 or 2] with (A2) a polythiol compound of formula II [R4 is 2W10C (OH-containing) polyvalent aliphatic hydrocarbon residue; p is 1W4] and/or formula III [R5 is 2W20C (OH-containing) aliphatic polyhydric alcohol residue; q is 1 or 2; r is 1W4] in the presence of a basic catalyst at a functional group equivalent ratio (A1/A2) of 2W10 with (B) ≤50wt.% of a polyfunctional vinyl monomer, e.g. the compound of formula such as bis(β-acryloyloxyethyl)phthalate.