The objects of the present invention are to provide a process for producing crystals of compound (I) easily in an industrial scale, and a stable lyophilized composition of compound (I). It has been found that subjecting an aqueous solution containing compound (I) and an inorganic salt such as sodium chloride to lyophilization yields a lyophilized composition having good storage stability wherein the compound (I) is crystalline, and further that crystals of compound (I) can be obtained from said aqueous solution without lyophilization.
