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 not ) of 11.6 degree , 17.2 degree , 17.8 degree , 23.3 degree , 27.1 degree and 29.3 degree in powder X-ray diffraction.