The purpose of the present invention is to provide stable crystals of (S)-1-(3-(4-amino-3-((3,5-dimethoxyphenyl)ethynyl)-1H-pyrazolo[3,4-d]pyrimidine-1-yl)-1-pyrrolidinyl)-2-propene-1-one, which is useful as an antitumor agent, having excellent oral absorbability and high chemical purity, the crystals being suitable for mass production. The present invention provides crystals of (S)-1-(3-(4-amino-3-((3,5-dimethoxyphenyl)ethynyl)-1H-pyrazolo[3,4-d]pyrimidine-1-yl)-1-pyrrolidinyl)-2-propene-1-one exhibiting at least three characteristic peaks for which the diffraction angle (2θ±0.2°) in the X-ray powder diffraction spectrum is selected from 9.5°, 14.3°, 16.7°, 19.1°, 20.8°, 21.9°, and 25.2°. The present invention also provides crystals of (S)-1-(3-(4-amino-3-((3,5-dimethoxyphenyl)ethynyl)-1H-pyrazolo[3,4-d]pyrimidine-1-yl)-1-pyrrolidinyl)-2-propene-1-one which are crystals exhibiting at least seven characteristic peaks for which the diffraction angle (2θ±0.2°) in the X-ray powder diffraction spectrum is selected from 13.5°, 17.9°, 19.5°, 20.6°, 22.0°, 22.6°, 23.3°, 23.7°, and 24.2°.
