A method for detecting gas leakage from a cargo tank is provided to detect whether a secondary barrier has defects or not according to the detection amount of inert gas. A method for detecting gas leakage from a cargo tank comprises the steps of: maintaining the same pressure in IS(Insulation Space) and IBS(Inter Barrier Space) of the cargo tank(a); injecting inert gas into any one of the IS and IBS in the same pressure condition(b); detecting a leakage of the inert gas from the other of the IS and IBS(c); and detecting whether a secondary barrier has defects according to the detection amount of the inert gas(d).
