"The sulfonium salt represented by the formula (4) can be synthesized for example by 1) a method in which a thioxanthone compound represented by a general formula (8) ##STR5## (wherein each of R1 to R5 is a group selected from a hydrogen atom, a hydroxyl group, a C1 -C15 aliphatic group, a halogen atom, a nitro group, an alkoxy group, an ester group and a phenyl group) is allowed to react with a su" . . . .