Photopolymerization initiator composition and photopolymerizable composition

A photopolymerization initiator composition containing

• (A) a sulfonium organoboron complex or oxosulfonium organoboron complex of the formula (1), ##STR13## wherein R.sup.1 is benzyl, substituted benzyl, phenacyl, substituted phenacyl, aryloxy, substituted aryloxy, alkenyl or substituted alkenyl, each of R.sup.2 and R.sup.3 is independently benzyl, substituted benzyl, phenacyl, substituted phenacyl, aryloxy, substituted aryloxy, alkenyl, substituted alkenyl, alkyl, substituted alkyl, aryl, substituted aryl, aralkyl, substituted aralkyl, alkynyl, substituted alkynyl, alicyclic, substituted alicyclic, alkoxy, substituted alkoxy, alkylthio, substituted alkylthio, amino, substituted amino, arylthio or substituted arylthio, or R.sup.2 and R.sup.3 bond to each other to form a cyclic structure, R.sup.4 is oxygen or a lone pair of electrons, and each of R.sup.5, R.sup.6, R.sup.7 and R.sup.8 is independently alkyl, substituted alkyl, aryl, substituted aryl, aralkyl, substituted aralkyl, alkenyl or substituted alkenyl, provided that not all of R.sup.5, R.sup.6, R.sup.7 and R.sup.8 are aryl or substituted aryl groups, and
• (B) a sensitizer selected from the group consisting of a benzophenone derivative, a triazine derivative having a trihalogenomethyl group and 2,4-diethylthioxanthone.