A photocationic polymerization initiator comprising (A) a photo acid-generating compound and (B) a fused polycyclic aromatic compound, wherein said fused polycyclic aromatic compound has a molecular structure in which a fused aromatic ring is further condensed with a non-aromatic ring, and at least one of the atoms constituting said non-aromatic ring directly bonded to a common atom in the fused aromatic ring and the non-aromatic ring, is a saturated carbon atom, said saturated carbon atom having at least one hydrogen atom. Upon blending the photopolymerizable monomer with the photocationic polymerization initiator, there is obtained a photocationically polymerizable composition having a high sensitivity for the visible light rays, which is useful for dental applications, exhibiting a sufficiently high curing rate and offering sufficient depth of curing upon the irradiation with light for only a short period of time.