The purpose of the invention is to provide crystals of (S,E)-2-(2,6-dichlorobenzamido)-5-[4-(methyl-pyrimidin-2- ylamino)phenyl]pent-4-enoic acid having excellent chemical and physical stability, and the pharmaceutical use thereof. The invention provides crystals of (S,E)-2-(2,6- dichlorobenzamido-5-[4-(methyl-pyrimidin-2- ylamino)phenyl]pent-4-enoic acid.