A blow molding device that supplies a pressurized liquid into a bottomed tubular preform fitted to a mold used for blow molding and molds the preform into a shape conforming to a cavity of the mold, the blow molding device comprising: a blow nozzle fitted to a mouth tubular portion of the preform; a sealing body configured to open and close the blow nozzle; a pressurized liquid supply unit connected to the blow nozzle via a liquid flow path; characterized in that the blow molding device further comprises: a cylinder used for conversion to air for an outside of a mouth, the cylinder having one end that is in branch connection with the liquid flow path and another end that is in connection with air space defined for pressuring the outside of the mouth and surrounding an outer circumference of the mouth tubular portion (PFb) of the preform fitted to the mold; an opening-closing valve of the cylinder, which is used for conversion to air for the outside of the mouth, disposed between the liquid flow path and the cylinder; an opening-closing valve used for pressurizing air outside the mouth and disposed between the air space, which is defined for pressuring the outside of the mouth, and the cylinder, which is used for conversion to air for the outside of the mouth; an accumulator that is in branch connection with the liquid flow path; and an opening-closing valve of the accumulator that is disposed between the liquid flow path and the accumulator, wherein, the pressurized liquid supply unit being configured to be actuated in a state where the sealing body and the opening-closing valve used for pressurizing air outside the mouth are closed and where the opening-closing valve of the cylinder used for conversion to air for the outside of the mouth, is opened, the opening-closing valve of the cylinder being configured to be subsequently closed, pressure being accumulated in advance in the cylinder, and pressure being also accumulated in the accumulator in advance, with the opening-closing valve of the accumulator being closed, and when the pressurized liquid is supplied into the preform through the blow nozzle by actuating the pressurized liquid supply unit and by opening the sealing body, the opening-closing valve used for pressurizing air outside the mouth is configured to be opened while the opening-closing valve of the cylinder, which is used for conversion to air for the outside of the mouth, is configured to remain closed to thereby allow the accumulated pressure within the cylinder to pressurize the air space, which is defined for pressuring the outside of the mouth, and the opening-closing valve of the accumulator is configured to be opened to thereby apply the accumulated pressure within the accumulator to the liquid supplied into the preform. a blow nozzle fitted to a mouth tubular portion of the preform; a sealing body configured to open and close the blow nozzle; a pressurized liquid supply unit connected to the blow nozzle via a liquid flow path; a cylinder used for conversion to air for an outside of a mouth, the cylinder having one end that is in branch connection with the liquid flow path and another end that is in connection with air space defined for pressuring the outside of the mouth and surrounding an outer circumference of the mouth tubular portion (PFb) of the preform fitted to the mold; an opening-closing valve of the cylinder, which is used for conversion to air for the outside of the mouth, disposed between the liquid flow path and the cylinder; an opening-closing valve used for pressurizing air outside the mouth and disposed between the air space, which is defined for pressuring the outside of the mouth, and the cylinder, which is used for conversion to air for the outside of the mouth; an accumulator that is in branch connection with the liquid flow path; and an opening-closing valve of the accumulator that is disposed between the liquid flow path and the accumulator, wherein, the pressurized liquid supply unit being configured to be actuated in a state where the sealing body and the opening-closing valve used for pressurizing air outside the mouth are closed and where the opening-closing valve of the cylinder used for conversion to air for the outside of the mouth, is opened, the opening-closing valve of the cylinder being configured to be subsequently closed, pressure being accumulated in advance in the cylinder, and pressure being also accumulated in the accumulator in advance, with the opening-closing valve of the accumulator being closed, and when the pressurized liquid is supplied into the preform through the blow nozzle by actuating the pressurized liquid supply unit and by opening the sealing body, the opening-closing valve used for pressurizing air outside the mouth is configured to be opened while the opening-closing valve of the cylinder, which is used for conversion to air for the outside of the mouth, is configured to remain closed to thereby allow the accumulated pressure within the cylinder to pressurize the air space, which is defined for pressuring the outside of the mouth, and the opening-closing valve of the accumulator is configured to be opened to thereby apply the accumulated pressure within the accumulator to the liquid supplied into the preform.