To provide a fluid leakage detector capable of detecting leakage of inert gas filled in a storage case of a high-voltage electric power source part in an inertia navigation system of an aircraft, and capable of preventing the inert gas in the storage case from being hardly replaced with air even when landings and flights of the aircraft are repeated.

An inner space of a bellows 4 is communicated with an inner space of an airtight container 2. A linear displacement gage 6 detects a displacement of an end face plate 5 via a detecting rod 7. The bellows 4 is contracted under atmospheric pressure atmosphere as shown in Fig.(a), and is expanded under reduced pressure atmosphere as shown in Fig.(b). A leakage amount of the inert gas from the airtight container 2 is estimated based on a change in a measured value measured by the linear displacement gage 6. The bellows 4 is contracted and expanded in response to a pressure difference between pressures in its inside and outside, the inside pressure of the airtight container 2 is atmospheric pressure when under the atmospheric pressure atmosphere, and the inert gas in the airtight container 2 is precluded thereby from being replaced with the air containing moisture to restrain insulation pressure-proofness of the high-voltage electric power source part from being deteriorated.