An object of the present invention is to provide 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, which is useful as an antitumor agent, the crystal being stable, excellent in oral absorbability, highly chemically pure, and suitable for mass production. The present invention 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 an X-ray powder diffraction spectrum containing at least three characteristic peaks at diffraction angles (2θ±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 an X-ray powder diffraction spectrum containing at least seven characteristic peaks at diffraction angles (2θ±0.2°) selected from 13.5°, 17.9°, 19.5°, 20.6°, 22.0°, 22.6°, 23.3°, 23.7°, and 24.2°.