PURPOSE: To detect gas leakages externally while an accumulator is being operated.

CONSTITUTION: An acceleration sensor 16 is disposed to the outer side of the cylinder 7 of an accumulator 4 so as to be connected to a frequency analyzing means 17, and the natural frequency of the cylinder 7 is outputted to an operating means 20. A temperature sensor 18 is arranged in the vicinity of the cylinder 7 so as to allow surrounding temperature to be outputted to the operating means 20. The operating means 20 judges the detection of gas leakages based on a relation between natural frequencies and surrounding temperatures at the time of no gas leakage out of those natural frequencies and surrounding temperatures which are stored in a memory means 20, and the detection of the gas leakage is thereby outputted to an outputting means 21.