Water level measuring device for pressure suppression pool

The present invention provides a device for measuring a water level of a device in which the water level tends to be changed such as a pressure suppression pool or a suppression pool in a nuclear power plant. Namely, a water level detection unit of the present invention comprises (1) a cladding tube filled with an insulation material, (2) a heating device having heating wires disposed to the cladding tube and (3) a thermocouple having a first junction disposed in a region of the heating device and a second junction disposed out of the region of the heating device. Then, a plurality of detection units are disposed in the pressure suppression pool so as to vertically offset the cladding tubes. With such a constitution, when the detection portion of the water level detection unit is in a low temperature water, there is no temperature difference between the two junction, which is judged that the water level is present above the detection portion. When the detection portion is out of low temperature water, temperature difference is caused between two junction, which is judged that water level is present below the detection portion. (I.S.)