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 (2θ ± 0.1˚) of 11.6˚, 17.2˚, 17.8˚, 23.3˚, 27.1˚ and 29.3˚ in powder X-ray diffraction.
