A method for detection of a leak from a sealed, thermally insulating tank for liquid gas, said tank comprising a sealing membrane (14, 15) surrounding the liquid gas, the sealing membrane being surrounded by an insulation space (16, 17) which separates the sealing membrane from a support wall, which itself is sealed, the insulation space being filled with solid, thermally insulating materials and an inert gas, the insulation space being provided with at least one injection duct (21, 22) and at least one extraction duct (23, 24) in order to inject and extract the inert gas, the detection method comprising the following steps: determining (921, 923) a first variation of mass of inert gas (ΔM1, ΔM3) between a first moment and a second moment, the first variation corresponding to the resultant of the total masses of inert gas added by the injection duct and removed by the extraction duct, between the first moment and the second moment; the method for detection being characterized in that it moreover comprises the following steps: determining a first mass of inert gas in the insulation space at the first moment and a second mass of inert gas in the insulation space at the second moment by means of measurements of the pressure and temperature in a free volume of the insulation space; calculating (922, 924) a second variation of mass of inert gas (ΔM2, ΔM4) between the first moment and the second moment, corresponding to the difference between the second mass and the first mass of inert gas; and comparing (931, 932) the first variation of mass of inert gas with the second variation of mass of inert gas, and triggering an alarm if a difference (E1, E2) between the first variation and the second variation of mass of inert gas is greater than a first threshold. determining (921, 923) a first variation of mass of inert gas (ΔM1, ΔM3) between a first moment and a second moment, the first variation corresponding to the resultant of the total masses of inert gas added by the injection duct and removed by the extraction duct, between the first moment and the second moment; the method for detection being characterized in that it moreover comprises the following steps: determining a first mass of inert gas in the insulation space at the first moment and a second mass of inert gas in the insulation space at the second moment by means of measurements of the pressure and temperature in a free volume of the insulation space; calculating (922, 924) a second variation of mass of inert gas (ΔM2, ΔM4) between the first moment and the second moment, corresponding to the difference between the second mass and the first mass of inert gas; and comparing (931, 932) the first variation of mass of inert gas with the second variation of mass of inert gas, and triggering an alarm if a difference (E1, E2) between the first variation and the second variation of mass of inert gas is greater than a first threshold. determining a first mass of inert gas in the insulation space at the first moment and a second mass of inert gas in the insulation space at the second moment by means of measurements of the pressure and temperature in a free volume of the insulation space; calculating (922, 924) a second variation of mass of inert gas (ΔM2, ΔM4) between the first moment and the second moment, corresponding to the difference between the second mass and the first mass of inert gas; and comparing (931, 932) the first variation of mass of inert gas with the second variation of mass of inert gas, and triggering an alarm if a difference (E1, E2) between the first variation and the second variation of mass of inert gas is greater than a first threshold.