A sulfonium salt formed of a sulfonium cation selected from a group indicated by general formulas,, and described below and a gallate anion represented by formula (a). [in formula, R 1 to R 4 each independently represent an alkyl group, a hydroxy group, an alkoxy group, an alkylcarbonyl group, an arylcarbonyl group, an alkoxycarbonyl group, an aryloxycarbonyl group, an arylthiocarbonyl group, an acyloxy group, an arylthio group, an alkylthio group, an aryl group, a heterocyclic hydrocarbon group, an aryloxy group, an alkylsulfinyl group, an arylsulfinyl group, an alkylsulfonyl group, an arylsulfonyl group, a hydroxy (poly) alkyleneoxy group, an optionally substituted silyl group, an optionally substituted amino group, a cyano group, a nitro group, or a halogen atom; a, b, c, and d each represent the number of occurrences of each of R 1 to R 4; a represents an integer of 1 to 5; c and d represent an integer of 0 to 5; and b represents an integer of 0 to 4.] [in formula, R 5 to R 8 each independently represent an alkyl group, a hydroxy group, an alkoxy group, an alkylcarbonyl group, an arylcarbonyl group, an alkoxycarbonyl group, an aryloxycarbonyl group, an arylthiocarbonyl group, an acyloxy group, an arylthio group, an alkylthio group, an aryl group, a heterocyclic hydrocarbon group, an aryloxy group, an alkylsulfinyl group, an arylsulfinyl group, an alkylsulfonyl group, an arylsulfonyl group, a hydroxy (poly) alkyleneoxy group, an optionally substituted silyl group, an optionally substituted amino group, a cyano group, a nitro group, or a halogen atom; e, f, g, and h each represent the number of occurrences of each of R 5 to R 8; e and g represent an integer of 0 to 4; f represents an integer of 0 to 3; h represents an integer of 0 to 5; x represents an optionally substituted phenyl group or an optionally substituted thioxanthonyl group; Y represents -O-, -S-, -SO-, -SO 2 - or -CO-.] [in formula, n represents an integer of 1 to 3; R 9 to R 10 each independently represent an alkyl group, a hydroxy group, an alkoxy group, an alkylcarbonyl group, an arylcarbonyl group, an alkoxycarbonyl group, an aryloxycarbonyl group, an arylthiocarbonyl group, an acyloxy group, an arylthio group, an alkylthio group, an aryl group, a heterocyclic hydrocarbon group, an aryloxy group, an alkylsulfinyl group, an arylsulfinyl group, an alkylsulfonyl group, an arylsulfonyl group, a hydroxy(poly)alkyleneoxy group, an optionally substituted silyl group, an optionally substituted amino group, a cyano group, a nitro group, or a halogen atom; i and j each represent the number of occurrences of each of R 9 to R 10; i and j represent an integer of 0 to 4; Q represents single bond, -O- or -S-; 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, a hydroxy group, an alkoxy group, an alkylcarbonyl group, an arylcarbonyl group, an alkoxycarbonyl group, an aryloxycarbonyl group, an arylthiocarbonyl group, an acyloxy group, an arylthio group, an alkylthio group, an aryl group, a heterocyclic hydrocarbon group, an aryloxy group, an alkylsulfinyl group, an arylsulfinyl group, an alkylsulfonyl group, an arylsulfonyl group, a hydroxy(poly)alkyleneoxy group, an optionally substituted silyl group, an optionally substituted amino group, a cyano group, a nitro group, or a halogen atom; Z represents an alkylene group having 2 to 6 carbon atoms or a group represented by general formula below; in formula, R 11 and R 12 represent a hydrogen atom, an alkyl group having 1 to 18 carbon atoms or an aryl group having 6 to 14 carbon atoms, and may be bonded to each other to form a ring structure.] [R 13 to R 16 each independently represent a phenyl group or a perfluoroalkyl group, and some of hydrogen atoms in these groups may be substituted with a group selected from a perfluoroalkyl group, a perfluoroalkoxy group, a nitro group, a cyano group, an acyl group and a halogen atom.]