A water filling system for a reactor water level gauge is provided for filling a reactor water level gauge instrumentation pipe in a reactor building with water and filling the reactor water level gauge with water even in an unexpected abnormal event where the reactor building is brought into a highly radioactive environment. The water filling system for a reactor water level gauge includes a water filling instrumentation pipe guided from the reactor water level gauge instrumentation pipe in the reactor building to an outside of the reactor building and filling the reactor water level gauge instrumentation pipe in the reactor building with water even in an unexpected abnormal event of a nuclear power plant.
