The present invention addresses the problem of producing a stabilized crystal of 5-chloro-6-[(2- iminopyrrolidin-1-yl)methyl]pyrimidine-2,4(1H,3H)-dione monohydrochloride. As a means for solving the problem, a crystal of 5-chloro-6-[(2-iminopyrrolidin-1-yl)methyl]pyrimidine-2,4(1H,3H)-dione monohydrochloride is provided. The crystal has characteristic peaks at diffraction angles (2[theta] +- 0.1 DEG ) of 11.6 DEG, 17.2 DEG, 17.8 DEG, 23.3 DEG, 27.1 DEG and 29.3 DEG in powder X-ray diffraction.本發明之課題在於獲得一種5-氯-6-[(2-亞胺基吡咯啶-1-基)甲基]嘧啶-2,4(1H,3H)-二酮‧單鹽酸鹽之穩定形結晶，作為其解決手段，提供一種於利用粉末X射線繞射所得之繞射角(2θ±0.1°)於11.6°、17.2°、17.8°、23.3°、27.1°、及29.3°處確認有特徵波峰之5-氯-6-[(2-亞胺基吡咯啶-1-基)甲基]嘧啶-2,4(1H,3H)-二酮‧單鹽酸鹽之結晶。
