Solvent (300 mL of acetonitrile and 200 mL of sulfolane) was added to 215 g (about 400 mmol) of crude crystal of A—SO3Na. Furthermore, phosphoryl trichloride (245.3 g, 1.6 mol) was added and reacted at 70° C. Furthermore, 5 mL of N,N-dimethylacetamide was added, a yellow suspension was stirred for 40 minutes at 71 to 73° C., and the reaction liquid was cooled to 3° C. 1 L of cold water was added at a rate so that the temperature of the reaction liquid did not exceed 10° C. The deposit was filtered and collected, washed with cold water, and recrystallized with 350 mL of toluene to obtain 153 of white crystal of 4-[4-(2,5-dichlorobenzoyl)phenoxy]benzene sulfonic acid chloride (A—SO3Cl). Yield depending on A was 87%. Melting point is 130.5 to 131.5° C.
