To regulate, in blow molding device using liquid, such as beverage, cosmetic product, and pharmaceutical product, filled in final product as pressurized liquid, head space in container which is filled with content liquid during molding to be predetermined volume, provided is blow molding device including blow nozzle configured to tightly communicate with mouth tubular portion of preform. In state where rod is disposed to be inserted into blow nozzle and where tip portion of rod is inserted into preform, pressurized liquid is supplied into preform via tubular introduction path formed by blow nozzle and rod, thereby expandingly shaping container in accordance with 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.