Substantially nonhygroscopic anhydrous crystals of compounds represented by the general formula: R2-CH2CONH-R1 (wherein R1 represents (un)substituted pyridyl or (un)substituted phenyl; and R2 represents (un)substituted 2-oxo-1-pyrrolidinyl). Drying the hydrous crystals of the above compounds at a temperature of 80 ~C or above under reduced pressure yields the corresponding anhydrous crystals with a weight gain of at most 1 % caused by moisture absorption after storage at a relative humidity of 83 % and a temperature of 25 ~C for 30 days. Medicines containing the above compounds in the form of anhydrous crystals are uniform in the active ingredient content.
