Disclosed is a cationically polymerizable composition which shows improved storage stability and whose polymerization initiation temperature or curing temperature is controlled. In the case where the composition is thermosetting, it yields a cured product of three-dimensional structure excellent in mechanical properties, electrical properties, adhesive properties, heat resistance, moisture resistance, chemical resistance, and the like. The cationically polymerizable composition comprises a cationically polymerizable compound selected from vinyl compounds, lactones, cyclic ethers, and the like, a heteropolyacid selected from phosphotungstic acid, phosphomolybdic acid, silicotungstic acid, silicomolybdic acid, and the like as a cationic polymerization initiator, and a stabilizer selected from heteropolyacid salts as an inhibitor of cationic polymerization.