Provided is a blow molding device (1) having: a mold (11) in which a preform (2) is disposed; a cylindrical nozzle (15) which is inserted into an opening portion (3) of the preform (2) from a lower end side; a pressurized fluid supplying unit (18) which supplies a pressurized incompressible fluid to a supply channel (16) connected to the nozzle (15) from an upper end side; a sealing body (20) which moves freely between a close position for closing the nozzle (15) and an open position for opening the nozzle (15); a plurality of suction openings (30) which are provided in the inner circumferential surface of the nozzle (15) and are arranged toward the axial direction of the nozzle (15); vertical openings (31) which are provided, extending along the axial direction, in the nozzle and which connect the plurality of suction openings (30) with one another; and a suction pump (33) which is connected to the vertical openings (31) via a suction path (32) and which provides the plurality of suction openings (30) a suction force for sucking the incompressible fluid.