THE PRESENT INVENTION PROVIDES A METHOD FOR PRODUCING A COMPOUND REPRESENTED BY FORMULA (A), THE METHOD COMPRISING THE STEPS OF: (A) MIXING A COMPOUND REPRESENTED BY FORMULA (B) WITH P—TOLUENESULFONIC ACID OR P—TOLUENESULFONIC ACID MONOHYDRATE AT LESS THAN 1 MOLAR EQUIVALENT WITH RESPECT TO THE COMPOUND REPRESENTED BY FORMULA (B) IN A SOLVENT UNDER HEATING (B) ADDING ADDITIONAL P-TOLUENESULFONIC ACID OR P-TOLUENESULFONIC ACID MONOHYDRATE TO THE MIXED SOLUTION UNDER COOLING, WHEREIN THE ADDITIONAL P—TOLUENESULFONIC ACID OR P-TOLUENESULFONIC ACID MONOHYDRATE IS ADDED IN SUCH AN AMOUNT THAT THE TOTAL MOLAR EQUIVALENT THEREOF WITH THE P-TOLUENESULFONIC ACID OR P-TOLUENESULFONIC ACID MONOHYDRATE OF STEP (A) IS 1 MOLAR EQUIVALENT OR MORE WITH RESPECT TO THE COMPOUND REPRESENTED BY FORMULA (B) OF STEP (A) AND (C) SUBSEQUENTLY ALLOWING THE MIXED SOLUTION TO CRYSTALLIZE TO OBTAIN THE COMPOUND REPRESENTED BY FORMULA (A). FIG. 2
