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? + 0.1o) of 11.6o, 17.2o, 17.8o, 23.3o, 27.1o and 29.3o in powder X-ray diffraction.
