A liquid blow molding apparatus for liquid blow molding a resin preform into a container having a prescribed shape. The apparatus includes a blow molding mold in which the preform is disposed; a blow nozzle that engages in an opening of the preform; a pressurized liquid supply that supplies a pressurized liquid to a supply path that is connected to the blow nozzle; and a seal member that is provided inside the supply path and opens and closes the supply path with respect to the blow nozzle. The pressurized liquid supply is configured to start supplying the pressurized liquid to the supply path after the seal member has begun to open.