Provided is a curable composition which has excellent thin-film curability and which can form, via the application of light and/or heat, a cured product having heat resistance and thermal yellowing resistance at excellent levels. This curable composition contains cationically curable compounds and a cationic-polymerization initiator. The cationically curable compounds include an epoxy compound (A) in an amount of 10 to 90 weight percent of the total weight of the cationically curable compounds, and an oxetane compound (B) in an amount of 5 to 40 weight percent of the total weight of the cationically curable compounds. The cationic-polymerization initiator includes an onium borate salt (C). The onium borate salt (C) preferably has an anionic moiety selected from [(C6H5)B(C6F5)3]−, [(C6H5)B((CF3CF2)2C6H2F)3]−, and [(C6H5C6H4)B(C6F5)3]− and a cationic moiety selected from arylsulfonium ions.