The purpose of the present invention is to provide a method with which it is possible to easily produce crystals of compound (I) on an industrial scale, and to provide a stable lyophilized composition of said compound (I). It was discovered that a lyophilized composition having exceptional storage stability and in which compound (I) is in crystalline form can be obtained by lyophilizing an aqueous solution that contains said compound (I) and an inorganic salt such as sodium chloride, and that crystals of said compound (I) can be obtained from said aqueous solution even without lyophilization.
