Provided are: a crystal form of (-)-(2-{[3-(5-fluoropyridin-2-yl)-1H-pyrazol-1-yl]methyl}-1,3-oxazinan-3-yl)[5-methyl-2-(2H-1,2,3-triazol-2-yl)phenyl]methanone, said crystal form being stable in the environment of usage thereof as a medicine; and a method for preparing the same. The present invention is crystals of (-)-(2-{[3-(5-fluoropyridin-2-yl)-1H-pyrazol-1-yl]methyl}-1,3-oxazinan-3-yl)[5-methyl-2-(2H-1,2,3-triazol-2-yl)phenyl]methanone which have at least one of characteristics (a) to (c): (a) in powder X-ray diffraction (Cu-K alpha), peaks appear at 2 theta=11.1 degrees, 12.5 degrees, 20.3 degrees and 24.2 degrees; (b) the melting point is 124 to 129 DEG C; and (c) in the infrared absorption spectrum, characteristic absorption bands appear at 1627cm-1, 1504 cm-1, 1226 cm-1, 1076 cm-1, 822 cm-1 and 783 cm-1. The present invention is also a method for preparing the crystals which is characterized by comprising: dissolving (-)-(2-{[3- (5-fluoropyridin-2-yl)-1H-pyrazol-1-yl]methyl}-1,3-oxazinan-3-yl)[5-methyl-2-(2H-1,2,3-triazol-2-yl)phenyl]methanone in a mixed solvent consisting of water and either a lower alcohol or acetone; and subjecting the solution to crystallization and then drying.