A simple and sensitive continuous monitoring system for the possible fuel failure of a pool-type nuclear reactor was successfully developed. Fuel failure detection (FFD) can be made by the detection of fission products (EP's) leaking from fuel element containers. In our system, fission rare gases (ィイD188ィエD1Kr, ィイD189ィエD1Kr, and ィイD1138ィエD1Xe) in the cooling water are collected with bubbled nitrogen gas, and then they are continuously led into a a vertical empty column (a decay and extraction chamber, φ100 mm × 2000 mm) , where they decay to yield daughter nuclides (ィイD188ィエD1Rb, ィイD189ィエD1Rb, and ィイD1138ィエD1Cs) which are extracted to aqueous phase. The radioactivity of daughter nuclides in the aqueous phase is continuously measured using a flow-through Ge-detector with excellent signal-to-noise ratios, because the collected fission rare gases are completely isolated from non-volatile background radioactive unclides in the reactor coolant. An only volatile component coming from the coolant is Ar-41, which is easily removed with bubbled nitrogen gas into the solution containing rubidium and cesium prior to detection. The newly developed continuous FFD system responded with so high sensitivity that a possible anomalous (twice) change in the concentration of FP rare gases in the coolant would be detected within about 30 minutes using the short-lived ィイD189ィエD1Kr-ィイD189ィエD1Rb pair. In addition to the above system, we tried to test, as the decay and extraction chambers, a packed column, a rotating drum, and a gas-washing bottle, which are less effective in the FFD.