Disclosed is hydrochloride of 6-amino-9-[(3R)-1-(2-butynoyl)-3-pyrrolidinyl]-7-(4- phenoxyphenyl)-7,9-dihydro-8H-purin-8-one and a crystal thereof. The compound is an inhibitor of Bruton&rsquo；s tyrosine kinase. It may be useful in the treatment of allergic disease, an autoimmune disease, an inflammatory disease, a thromboembolic disease, a bone-related disease, or cancer including non-Hodgkin&rsquo；s lymphoma.
