PROBLEM TO BE SOLVED: To provide a reactor water level measuring device capable of accurately measuring a water level in a reactor pressure vessel not only in normal operation but also in emergency and restoration.SOLUTION: The reactor water level measuring device comprises: a reference pressure line 4A and the like connected to a gas phase part of a reactor pressure vessel 1 through a condensation tank 3; a water level pressure line 5A and the like connected to a liquid phase part of the reactor pressure vessel 1; a differential pressure gauge 6 preset so that a water level h1 of the condensation tank 3 in normal operation is used as a reference; a balance pipe 11 which is branched from a branch part 9 of the reference pressure line 4A positioned outside a reactor containment vessel and connected to join the water level pressure line 5A, and which allows water on the upper end side of the branch part 9 of the reference pressure line 4A to flow out so that a water level becomes as high as the branch part 9 of
