The present invention provides a cocrystal of (S)-3-(1-((1-acryloylpyrrolidin-3-yl)oxy)isoquinolin-3-yl)-1H-1,2,4-triazol-5(4H)-one and an organic acid capable of forming a cocrystal with the compound. Specific cocrystals prepared are cocrystals with gentisic acid, salicylic acid, maleic acid, malonic acid, malic acid, mandelic acid and citric acid.