A steam-replacement deaeration method uses a nozzle having a double-tube structure of an inner first nozzle member and an outer second nozzle member. An insert part of the first nozzle member that is to be inserted into a bag has a flat cross-sectional configuration. The body of the second nozzle member has a circular cross-section and surrounds the insert part of the first nozzle member. At a first stage of steam injection, both the first and second nozzle members are inserted into the bag, and steam is injected from the spouts of both of them. Next, only the second nozzle member is removed from the bag, and the steam injection therefrom is stopped. While the steam injection from the first nozzle member being continued, the bag mouth is brought into a tensed state. Next, the first nozzle member is removed from the bag, and the bag is moved to the subsequent step with the bag mouth kept in the tensed state.