Provided is a crystal of hydrate of 4-(((6S,9S)-1-(benzylcarbamoyl)-2,9-dimethyl-4,7-dioxo-8-(quinolin-8-ylmethyl)octahydro-1H-pyrazino[2,1-c][1,2,4]triazin-6-yl)methyl)phenyl dihydrogen phosphate, showing a diffraction peak at a diffraction angle (2θ±0.2°) of 15.9°, in powder X-ray diffraction.