. . . "In illustratively describing the method 2), a sulfonium salt synthesized by the method 1), for example a halide compound represented by a formula (13) ##STR14## (wherein each of R1 to R5 is a group selected from a hydrogen atom, a C1 -C15 aliphatic group, a nitro group, an alkoxy group and a phenyl group, Z is as defined by the aforementioned formula (6) or (7) and A is a halogen atom), is allowed" .