The lyophilizate is dried under reducedpressure and in the presence of a dehydrating agent such as silica gel, phosphoric anhydride or the like, thereby to obtain the anhydrate of compound (I).(b) the compound (II) or its salt is dissolved in water or a mixture of water with one of the aforementioned hydrophilic organic solvents, followed by the addition of HX. After the reaction has been complete