PURPOSE: To provide a device for detecting leaked coolant of extremely small amount not detected heretofore in preferable response.

CONSTITUTION: This leakage detector 11 is connected through the wall of the storage container 2 for containing the container 3 of a reactor 1 to gas exhaust and return tubes 7 and 8, and consists of a dehumidifier 12, a filter 13 for removing dusts and mists, a flow meter 14, fission product gas collector 15, and a pump 16. This detector 11 also has a disturbing radiation ray density measuring circuit 30, and fission porduct gas density measuring circuit 13 to measure the radiation ray density from nuclides except for the fission product gas be applying the outputs from the radiated ray detectors 27, 28 provided in the vicinity of the dehumidifier 12 and the filter 13 and from the flow meter 14 to the circuit 30 to operate a warning circuit 32 when the difference between the output and the output from the circuit 31 is higher than predetermined value so as to warn an abnormality