A blow molding device for blow molding a bottomed tubular preform including a mouth tubular portion projecting at an upper end of the preform, the blow molding device comprising: a metal mold used for the blow molding; and a blow nozzle configured to tightly communicate with the mouth tubular portion when the preform is fitted to the metal mold, wherein in a state where a rod is disposed to be inserted into the blow nozzle and where a tip portion of the rod is inserted into the preform, a pressurized liquid is supplied into the preform via a tubular introduction path (Fi) formed by means of the blow nozzle and the rod, thereby expandingly shaping a container in accordance with a cavity of the metal mold, and when the container is shaped and the supply of the pressurized liquid is stopped, the rod is disinserted from the container, and according to a shape and an insert position of the tip portion of the rod, head space (Hs) of the shaped container that is filled with the liquid is regulated to be a predetermined volume, the rod comprising a stretching rod for vertically stretching the preform, characterized in that the stretching rod includes a tubular portion in a region extending from an upper end portion to a predetermined height position of the stretching rod, the tubular portion being provided, inside thereof, with a discharge path (Fd) extending vertically, and the stretching rod also including a horizontal hole provided below the tubular portion to extend through a tubular wall and communicate with the discharge path (Fd), the pressurized liquid is supplied into the preform via the tubular introduction path (Fi) formed by means of the blow nozzle and the stretching rod, thereby shaping the container in accordance with the cavity of the metal mold, and after the container is shaped and the supply of the pressurized liquid is stopped, pressurized air is supplied to the introduction path (Fi), thereby pressurizing the liquid remaining in a portion extending from the inside of the container to the introduction path (Fi), and thereby discharging a part of the remaining liquid via the horizontal hole of the stretching rod and the discharge path (Fd), and according to a height position at which the horizontal hole is located, the head space (Hs) of the shaped container that is filled with the liquid is regulated to be the predetermined volume.. a metal mold used for the blow molding; and a blow nozzle configured to tightly communicate with the mouth tubular portion when the preform is fitted to the metal mold, wherein in a state where a rod is disposed to be inserted into the blow nozzle and where a tip portion of the rod is inserted into the preform, a pressurized liquid is supplied into the preform via a tubular introduction path (Fi) formed by means of the blow nozzle and the rod, thereby expandingly shaping a container in accordance with a cavity of the metal mold, and when the container is shaped and the supply of the pressurized liquid is stopped, the rod is disinserted from the container, and according to a shape and an insert position of the tip portion of the rod, head space (Hs) of the shaped container that is filled with the liquid is regulated to be a predetermined volume, the rod comprising a stretching rod for vertically stretching the preform, characterized in that the stretching rod includes a tubular portion in a region extending from an upper end portion to a predetermined height position of the stretching rod, the tubular portion being provided, inside thereof, with a discharge path (Fd) extending vertically, and the stretching rod also including a horizontal hole provided below the tubular portion to extend through a tubular wall and communicate with the discharge path (Fd), the pressurized liquid is supplied into the preform via the tubular introduction path (Fi) formed by means of the blow nozzle and the stretching rod, thereby shaping the container in accordance with the cavity of the metal mold, and after the container is shaped and the supply of the pressurized liquid is stopped, pressurized air is supplied to the introduction path (Fi), thereby pressurizing the liquid remaining in a portion extending from the inside of the container to the introduction path (Fi), and thereby discharging a part of the remaining liquid via the horizontal hole of the stretching rod and the discharge path (Fd), and according to a height position at which the horizontal hole is located, the head space (Hs) of the shaped container that is filled with the liquid is regulated to be the predetermined volume..