A controller (31) closes valves (7, 9), opens valves (4, 12) and introduces water to be treated into a vacuum deaeration tank (1) using an introduction pump (3). Next, the controller (31) stops the introduction pump (3) when the water level in the vacuum deaeration tank (1) reaches a prescribed level and closes the valves (4, 12). Next, the controller 31 opens the valve (9) and carries out vacuum deaeration of the water to be treated using a vacuum pump (10). Next, the controller (31) stops the vacuum pump (10), closes the valve (9) and opens the vacuum deaeration tank (1) to atmospheric pressure by opening the valve (12). Next, the controller (31) closes the valve (12) and opens the valve (9). The controller (31) runs the vacuum pump (10) once again carries out vacuum deaeration of the water to be treated. Thereafter, the controller (31) carries out a final vacuum deaeration after repeating vacuum deaeration and opening to atmospheric pressure a prescribed number of times and terminates the treatment.
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