An object of the present invention is to provide acrystal of(S)-1-(3-(4-amino-3-((3,5-dimethoxyphenyl)ethynyl)-1H-pyrazolo[3,4-d]pyrimidin-1-yl)-1-pyrrolidinyl)-2-propen-1-one,which is useful as an antitumor agent, the crystal being stable,excellent in oral absorbability, highly chemically pure, andsuitable for mass production. The present invention provides acrystal of(S)-1-(3-(4-amino-3-((3,5-dimethoxyphenyl)ethynyl)-1H-pyrazolo[3,4-d]pyrimidin-1-yl)-1-pyrrolidinyl)-2-propen-1-onethat exhibits an X-ray powder diffraction spectrum containing atleast three characteristic peaks at diffraction angles (2.theta.+ -0.2°)selected from 9.5°, 14.3°, 16.7°, 19.1°,20.8°, 21.9°, and 25.2°.The present invention also provides a crystal of(S)-1-(3-(4-amino-3-((3,5-dimethoxyphenyl)ethynyl)-1H-pyrazolo[3,4-d]pyrimidin-1-yl)-1-pyrrolidinyl)-2-propen-1-one that exhibits anX-ray powder diffraction spectrum containing at least sevencharacteristic peaks at diffraction angles (2.theta.+ -0.2° ) selectedfrom 13.5°, 17.9°, 19.5°, 20.6°, 22.0°,22.6°, 23.3', 23.7°, and24.2°.
