Disclosed herein is a method of producing prasugrel hydrochloride (compound of formula (Ia)), comprising the steps of: i) chlorinating a compound represented by formula (III), by adding a chlorinating agent optionally dropwise thereto in a solvent； ii) reacting the resultant compound represented by formula (IV) with a compound represented by formula (V), wherein R represents a protecting group for a hydroxyl group, or a salt thereof in a solvent in the presence of a base； iii) acetylating the resultant compound represented by formula (II), wherein R has the same meaning as above, by reacting an acetylating agent therewith in a solvent in the presence of a base and an acylation catalyst； and iv) adding hydrochloric acid, optionally dropwise to the resultant compound of formula (I) in a solvent, thereby producing prasugrel hydrochloride as represented by formula (Ia)； in which in step i), the temperature during the addition, optionally dropwise, of the chlorinating agent is -20°C to 5°C and the reacting temperature after the addition, optionally dropwise, of the chlorinating agent is -20°C to 5°C.
