PROBLEM TO BE SOLVED: To provide a reactor water level measuring device which is capable of accurately measuring a water level inside a reactor pressure vessel in an emergency.SOLUTION: A reactor water level measuring device for measuring a water level inside a reactor pressure vessel 1 comprises: a reference pressure pipe 2 connected to a gas phase portion of the reactor pressure vessel 1; a water level pressure pipe 3 connected to a liquid phase portion of the reactor pressure vessel 1; a differential pressure gauge 4 which measures the differential pressure between a reference pressure introduced from the reference pressure pipe 2 and a water level pressure introduced from the water level pressure pipe 3, as a state quantity corresponding to the water level inside the reactor pressure vessel 1; and a balance pipe 8 which is connected to the reference pressure pipe 2 in such a manner that it branches off from a branching part 6 located outside the reactor pressure vessel 5. The balance pipe 8 is configured
