A passive auxiliary condensing apparatus of a nuclear power plant includes a steam generation unit configured to heat a water supplied thereto into a steam by a heat produced when operating a nuclear reactor, a water cooled heat exchange unit connected to the steam generation unit and configured to store a cooling water therein to condense the steam provided from the steam generation unit, and a steam-water separation tank including a first side connected to the water cooled heat exchange unit and a second side connected to the steam generation unit, wherein a mixture of a water and a steam provided from the water cooled heat exchange unit is separated into the water and the steam to provide only the water to the steam generation unit.