To effectively prevent occurrence of stretching unevenness during blow molding using a liquid as the pressure medium without significantly modifying the structure of a device, provided is a blow molding device including: a metal mold used for blow molding; and a blow nozzle that tightly communicates with a mouth tubular portion of a bottomed tubular preform, the liquid as the pressure medium being supplied into the preform through the blow nozzle. The blow nozzle is provided, at a lower end portion thereof, with an engaging tubular piece that is inserted into the mouth tubular portion, and the engaging tubular piece has a length by which a lower end of the engaging tubular piece extends further downward than a neck ring in a state where the blow nozzle is in communication with the mouth tubular portion, the neck ring being provided around a lower end of the mouth tubular portion.