(S)-1-(4-Chloro-5-isoquinolinesulfonyl)-3-(methylamino)pyrrolidine monohydrochloride and a crystal thereof, and a crystal of the aforementioned monohydrochloride having a major peak or peaks at one or more positions selected from the group consisting of positions where 2θs are about 13.9°, 21.5°, 21.7°, 22.4°, 22.8°, 24.5° and 35.0° in a powder X-ray diffraction spectrum, which have excellent properties as active ingredient of a medicament for prophylactic and/or therapeutic treatment of glaucoma and the like.