The purinone derivative 6-amino-9-[(3R)-1-(2-butynoyl)-3-pyrrolidinyl]-7-(4-phenoxyphenyl)-7,9-dihydro-8H-purin-8-one hydrochloride has Btk-selective inhibitory activity and, in addition to having excellent metabolic stability, it is a compound that exhibits a high level of solubility and absorption with respect to the free base and can be crystallized, hence it can serve as a therapeutic agent for diseases involving B cells and mast cells.