A blow molding device of the invention includes a pair of blow cavity split molds (50A, 50B), a chuck member (80) that chucks a mouth portion (20) and conveys a preform (10A), a blow core mold (70) that includes a tubular portion (71) which surrounds circumferences of the mouth portion and a flange (25) and introduces blow air into the preform, and a pair of projecting members (60A, 60B) which have a halved ring shape and are provided detachably on each upper surface of the pair of blow cavity split molds. Each of the pair of projecting members includes an airtight sealing portion (61) which is in contact with a lower end face of the tubular portion of the blow core mold, and a positioning portion (62) which positions the flange of the preform.