To regulate, in blow molding device using liquid, such as beverage, cosmetic product, and pharmaceutical product, filled in final product as pressurized liquid, an head space in a container which is filled with liquid during molding. The blow molding device including blow nozzle (4) configured to tightly communicate with the mouth tubular portion of preform( 31). The rod is disposed to be inserted into blow nozzle (4) and the tip portion of rod is inserted into the preform. Pressurized liquid is supplied into the preform via a tubular introduction path (Fi) formed by a blow nozzle and the rod (8a), thereby expandingly shaping container in accordance with the cavity of metal mold. When container is shaped and supply of pressurized liquid is stopped, rod is disinserted from container, and according to shape and insert position of tip portion of rod, head space of shaped container that is filled with liquid is regulated to be predetermined volume.A valve mechanism (Vm) is provided in an end portion on a downstream side of the introduction path (Fi) formed in the blow nozzle (4) for allowing the introduction path (Fi) to be opened and closed. A tubular rod-shaped shaft body (10a) is displaceably inserted and disposed in the blow nozzle (4) along an axis direction of the blow nozzle (4), and the stretching rod (8a) is inserted in the shaft body (10a) such that the stretching rod (8a) is slidable in a liquid tight manner, the shaft body (10a) and the stretching rod (8a) constituting a seal pin (10). The valve mechanism (Vm) is configured by bringing and releasing a tip portion of the seal pin (10) into and from the abutment against a seal stepped portion (6s) provided on an inner circumferential surface of the blow nozzle (4).