PURPOSE: To increase response property to secure a high sensitivity and at the same time ensure the steady maintenance of the gas flow rate with a simple means, by using the fiber layer of a prescribed composition for the collecting filter of the dust in the gas for a high-speed breeder.

CONSTITUTION: The joint end part between the pipelines 1 and 2 is used for the sodium leak detecting part 3 having an increased diameter. The differential pressure detector is formed at the part 3 to detect the filter blockade due to the sodium aerosol in the form of the increment of differential pressure between the up-stream and down-stream of the filter. Then the sodium is the gas flowing through the pipeline 1, the part 3 and the pipeline 2 is collected by the filter 7. The increment of differential pressure between the up-stream and down-stream of the filter 7 is detected by the differential pressure transmitter 10 to detect the leak of sodium. In this case, the filter 7 is formed with the fiber layer obtained by twining the fibers of the cellulose or the like having the fiber diameter of several μm W several tens μm.