A heat- or energy ray-curable composition comprising a cationically polymerizable compound and an acid generator containing an onium gallate salt represented by general formula below, wherein the gallate anion salt contains impurities in an amount of 4% or less: [in formula, R 1 to R 4 each independently represent an alkyl group having 1 to 18 carbon atoms or Ar, with the proviso that at least one of R 1 to R 4 is Ar; Ar represents an aryl group having 6 to 14 carbon atoms (excluding the number of carbon atoms of substituents below), and some of hydrogen atoms in the aryl group may be substituted with an alkyl group having 1 to 18 carbon atoms, a halogen atom-substituted alkyl group having 1 to 8 carbon atoms, an alkenyl group having 2 to 18 carbon atoms, an alkynyl group having 2 to 18 carbon atoms, an aryl group having 6 to 14 carbon atoms, a nitro group, a hydroxyl group, a cyano group, an alkoxy group or aryloxy group represented by -OR 6, an acyl group represented by R 7 CO-, an acyloxy group represented by R 8 COO-, an alkylthio group or arylthio group represented by -SR 9, an amino group represented by -NR 10 R 11, or a halogen atom; R 6 to R 9 each represent an alkyl group having 1 to 8 carbon atoms, or an aryl group having 6 to 14 carbon atoms; R 10 and R 11 each represent a hydrogen atom, an alkyl group having 1 to 8 carbon atoms, or an aryl group having 6 to 14 carbon atoms; E represents an n-valent element of Group 15 to Group 17 (IUPAC nomenclature); n represents an integer of 1 to 3; and R 5 is an organic group bonded to E, the number of R 5 s is n + 1, (n + 1) R 5 s may be the same or different, and two or more R 5 s may be linked together directly or through -O-, -S-, -SO-, -SO 2 -, -NH-, -CO-, -COO-, -CONH-, an alkylene group or a phenylene group to form a ring structure including the element E]. [in formula, R 1 to R 4 each independently represent an alkyl group having 1 to 18 carbon atoms or Ar, with the proviso that at least one of R 1 to R 4 is Ar; Ar represents an aryl group having 6 to 14 carbon atoms (excluding the number of carbon atoms of substituents below), and some of hydrogen atoms in the aryl group may be substituted with an alkyl group having 1 to 18 carbon atoms, a halogen atom-substituted alkyl group having 1 to 8 carbon atoms, an alkenyl group having 2 to 18 carbon atoms, an alkynyl group having 2 to 18 carbon atoms, an aryl group having 6 to 14 carbon atoms, a nitro group, a hydroxyl group, a cyano group, an alkoxy group or aryloxy group represented by -OR 6, an acyl group represented by R 7 CO-, an acyloxy group represented by R 8 COO-, an alkylthio group or arylthio group represented by -SR 9, an amino group represented by -NR 10 R 11, or a halogen atom; R 6 to R 9 each represent an alkyl group having 1 to 8 carbon atoms, or an aryl group having 6 to 14 carbon atoms; R 10 and R 11 each represent a hydrogen atom, an alkyl group having 1 to 8 carbon atoms, or an aryl group having 6 to 14 carbon atoms; E represents an n-valent element of Group 15 to Group 17 (IUPAC nomenclature); n represents an integer of 1 to 3; and R 5 is an organic group bonded to E, the number of R 5 s is n + 1, (n + 1) R 5 s may be the same or different, and two or more R 5 s may be linked together directly or through -O-, -S-, -SO-, -SO 2 -, -NH-, -CO-, -COO-, -CONH-, an alkylene group or a phenylene group to form a ring structure including the element E].