A photopolymerization initiator comprising (A) an alpha -
diketone compound such as camphorquinone, (B1) an
aliphatic amine compound such as
triethanolamine or N-methyldiethanolamine, (B2) an
aromatic amine compound such as ethyl p-dimethylaminobenzoate or N,N-dimethyl p-
toluidine, and (C) a
triazine compound substituted with a trihalomethyl group, such as 2,4,6-
tris(trichloromethyl)-s-
triazine or 2-phenyl-4,6-bis(trichloromethyl)-s-
triazine. The photopolymerization initiator remains stable against the environmental light such as the indoor illumination light from a
fluorescent lamp or the dental light illuminating the interior of the
oral cavity, is excellently handled, is quickly cured by the
irradiation with an intense light emitted from a light irradiator for
polymerization, provides a cured body having excellent properties, can be stably preserved even at high temperatures, and is very useful as a
dental composite resin.