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 (2?+0.2o) in the X-ray powder diffraction spectrum is selected from 9.5o, 14.3o, 16.7o, 19.1o, 20.8o, 21.9o, and 25.2o. 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 (2?+0.2o) in the X-ray powder diffraction spectrum is selected from 13.5o, 17.9o, 19.5o, 20.6o, 22.0o, 22.6o, 23.3o, 23.7o, and 24.2o.
