A deaerator includes a tank, a spray unit, a steam supply unit, a bleed unit and a discharge pipe. The spray unit is disposed at an upper portion of the tank and configured to supply water to the tank. The steam supply unit is disposed inside the tank to supply steam to the tank. The bleed unit is disposed at the upper portion of the tank and adjacent to the spray unit. The bleed unit is configured to bleed air from an inside of the tank. The discharge pipe is configured to discharge water without air to an outside of the tank.