A process for preparing a paroxetine hydrochloride hydrate, comprising reacting (3S, 4R)-1-tert-butoxycarbonyl-4-(4-fluorophenyl)-3-[(3,4-methyl- ene-dioxy)phenoxymethyl]piperidine with hydrogen chloride in the presence of water, and allowing crystals to separate out from the resulting reaction mixture in the presence of water.