A hydrochloride of 6-amino-9-[(3R)-1-(2-butynoyl)-3-pyrrolidinyl]-7-(4- phenoxyphenyl)-7,9-dihydro-8H-purin-8-one, which is a purinone derivative, is a compound having a Btk-selective inhibitory activity and excellent metabolism stability, capable of solubilizing and absorbing a free base satisfactorily, and capable of being crystallized. Therefore, the compound can be used as a therapeutic agent for diseases associated with a B cell or a mast cell.
