A photopolymerization method in which an ink composition including (A) a polymerizing compound having an ethylenic unsaturated group, (B) a photopolymerization initiator represented by Formula (1) described below, and (C) a hydrogen donor having a structure in which a nitrogen atom is directly bonded to an aromatic ring, in which the aromatic ring has an electron-withdrawing group or the nitrogen atom constitutes a hetero ring, is photopolymerized under acidic conditions, in Formula (1), each of R1 to R30 represents a hydrogen atom, a halogen atom, an alkyl group, or an alkoxy group.