Pressure is stored in advance in a cylinder (35) for exchanging air outside the mouth to close an opening/closing valve (V3) of the cylinder for air exchange outside the mouth. When supplying pressurized liquid (L) to the preform (PF) through the blow nozzle (13) by operating a plunger pump (31) and opening a seal (15), the pressure stored in the cylinder (35) for air exchange outside the mouth is added to the liquid (L) that is being supplied to the preform (PF) by opening the opening/closing valve (V3) of the cylinder for air exchange outside the mouth.
