Disclosed is a compound composed of an easily-handleable crystal form and having storage stability that is practical enough to be used as an active ingredient of a pharmaceutical product. Specifically disclosed is 6-[(3R)-3-aminopiperidin-1-yl]-5-(2-chloro-5-fluorobenzyl)-1,3-dimethyl-1H-pyrrolo[3,2-d]pyrimidine-2,4(3H,4H)-dione monohydrochloride hemihydrate.