An optically active 2-hydroxytetrahydrothienopyridine derivative or a pharmaceutically acceptable salt thereof, wherein said derivative is (S)-methyl 2-(2-acetoxy-6,7-dihydrothieno[3,2-c]pyridin-5(4H)-yl)-2-(2-chlorophenyl)-acetate, wherein said pharmaceutically acceptable salt is a hydrochloric acid salt, a hydrobromic acid salt, a sulfuric acid salt, a citric acid salt, a tartaric acid salt, a phosphoric acid salt, a lactic acid salt, an acetic acid salt, a maleic acid salt, a fumaric acid salt, a malic acid salt, a mandelic acid salt, a methanesulfonic acid salt, a benzenesulfonic acid salt, a p-toluenesulfonic acid salt, a pamoic acid salt, an oxalic acid salt, or a succinic acid salt.