A blow molding device for blow-molding a preform which includes a mouth portion, a body portion, and a flange provided between the mouth portion and the body portion into a container, the blow molding device comprising: a pair of blow cavity split molds (50A, 50B); a chuck member that is designed to chuck the mouth portion and to convey the preform to the pair of blow cavity split molds (50A, 50B) which are openable; a blow core mold that includes a tubular portion, wherein circumferences of the mouth portion and the flange of the preform are surroundable by the tubular portion and the tubular portion is designed to introduce blow air into the preform, after the chuck member is retracted from the preform which is disposable in the pair of blow cavity split molds (50A, 50B) which are closable; and a pair of projecting members (60A, 60B) which have a halved ring shape and are provided detachably on each upper surface (50a, 50b) of the pair of blow cavity split molds (50A, 50B), wherein each of the pair of projecting members (60A, 60B) includes: a positioning portion which is designed to position the flange of the preform when the pair of blow cavity split molds (50A, 50B) are closed, and wherein each of the pair of projecting members (60A, 60B) further includes an airtight sealing portion which is contactable with a lower end face of the tubular portion of the blow core mold and wherein the positioning portion is formed on an inner circumferential surface of the projecting member (60A, 60B), characterized in that the airtight sealing portion has a lower wear resistance than that of the lower end face of the tubular portion, and the chuck member adopts a type in which two chucking pieces are normally biased in a closing direction by a spring, and when the chuck member is detached from the mouth portion of the preform, the spring is elastically deformed as the chuck member is retracted, whereby the chuck member is drawn from the mouth portion, and as this occurs, although external force is applied to the preform, since the flange of the preform is positioned by the positioning portions, the preform is not moved to be decentered. a pair of blow cavity split molds (50A, 50B); a chuck member that is designed to chuck the mouth portion and to convey the preform to the pair of blow cavity split molds (50A, 50B) which are openable; a blow core mold that includes a tubular portion, wherein circumferences of the mouth portion and the flange of the preform are surroundable by the tubular portion and the tubular portion is designed to introduce blow air into the preform, after the chuck member is retracted from the preform which is disposable in the pair of blow cavity split molds (50A, 50B) which are closable; and a pair of projecting members (60A, 60B) which have a halved ring shape and are provided detachably on each upper surface (50a, 50b) of the pair of blow cavity split molds (50A, 50B), wherein each of the pair of projecting members (60A, 60B) includes: a positioning portion which is designed to position the flange of the preform when the pair of blow cavity split molds (50A, 50B) are closed, and wherein each of the pair of projecting members (60A, 60B) further includes an airtight sealing portion which is contactable with a lower end face of the tubular portion of the blow core mold and wherein the positioning portion is formed on an inner circumferential surface of the projecting member (60A, 60B), characterized in that the airtight sealing portion has a lower wear resistance than that of the lower end face of the tubular portion, and the chuck member adopts a type in which two chucking pieces are normally biased in a closing direction by a spring, and when the chuck member is detached from the mouth portion of the preform, the spring is elastically deformed as the chuck member is retracted, whereby the chuck member is drawn from the mouth portion, and as this occurs, although external force is applied to the preform, since the flange of the preform is positioned by the positioning portions, the preform is not moved to be decentered. a positioning portion which is designed to position the flange of the preform when the pair of blow cavity split molds (50A, 50B) are closed, and wherein each of the pair of projecting members (60A, 60B) further includes an airtight sealing portion which is contactable with a lower end face of the tubular portion of the blow core mold and wherein the positioning portion is formed on an inner circumferential surface of the projecting member (60A, 60B), characterized in that the airtight sealing portion has a lower wear resistance than that of the lower end face of the tubular portion, and the chuck member adopts a type in which two chucking pieces are normally biased in a closing direction by a spring, and when the chuck member is detached from the mouth portion of the preform, the spring is elastically deformed as the chuck member is retracted, whereby the chuck member is drawn from the mouth portion, and as this occurs, although external force is applied to the preform, since the flange of the preform is positioned by the positioning portions, the preform is not moved to be decentered.