A detection method of an airtight failure in a working-gas circulating type gas engine, said working-gas circulating type gas engine leads fuel gas, oxidizer gas, and working gas to a combustion chamber and burns said fuel gas in the combustion chamber to obtain motive power, and said working-gas circulating type gas engine comprises: a fuel gas supplying means to supply said fuel gas, an oxidizer gas supplying means to supply said oxidizer gas, a working-gas supplying means to supply said working gas, a sealed circulation passage which circulates burnt gas emitted from an exhaust port which communicate to said combustion chamber to an intake port which communicates to said combustion chamber circulate therethrough, and a combustion product removing means to separate and remove combustion product produced by combustion of said fuel gas from said burnt gas, wherein said working-gas circulating type gas engine further comprises: a gas quantity detecting means to detect gas quantity in said circulation passage, and an airtightness judging means (S405, S406, S407, S408, S415, S416, S417, S418) to judge the airtightness of said circulation passage, and characterized in that said detection method includes: a surplus gas supplying step (S403, S413) in which at least one of said fuel gas, said oxidizer gas, and said working gas in an extra predetermined quantity 1 (Ps) in addition to the quantity needed for compensating for the decline associated with operation of the engine is supplied as surplus gas into said circulation passage, a circulating gas increase detecting step (S404, S414) in which a circulating gas increase (Pd) which is an increment of the quantity of gas in said circulation passage before and after said surplus gas supplying step (S403, S413) is detected by said gas quantity detecting means, and an airtightness judging step (S405, S406, S407, S408, S415, S416, S417, S418) in which it is judged that there is an airtight failure in said working-gas circulating type gas engine (S408, S418) when the absolute value of a quantity deviation of circulation gas (|ΔP|, S405, S415) which is the difference between said predetermined quantity 1 (Ps) and said circulating gas increase (Pd) is a predetermined quantity 2 or more (S406: No, S416: No), while it is judged that there is no airtight failure in said working-gas circulating type gas engine (S407, S417) when the absolute values of said quantity deviation of circulation gas (|ΔP|) is less than said predetermined quantity 2 (S406: Yes, S416: Yes). a fuel gas supplying means to supply said fuel gas, an oxidizer gas supplying means to supply said oxidizer gas, a working-gas supplying means to supply said working gas, a sealed circulation passage which circulates burnt gas emitted from an exhaust port which communicate to said combustion chamber to an intake port which communicates to said combustion chamber circulate therethrough, and a combustion product removing means to separate and remove combustion product produced by combustion of said fuel gas from said burnt gas, a gas quantity detecting means to detect gas quantity in said circulation passage, and an airtightness judging means (S405, S406, S407, S408, S415, S416, S417, S418) to judge the airtightness of said circulation passage, and characterized in that a surplus gas supplying step (S403, S413) in which at least one of said fuel gas, said oxidizer gas, and said working gas in an extra predetermined quantity 1 (Ps) in addition to the quantity needed for compensating for the decline associated with operation of the engine is supplied as surplus gas into said circulation passage, a circulating gas increase detecting step (S404, S414) in which a circulating gas increase (Pd) which is an increment of the quantity of gas in said circulation passage before and after said surplus gas supplying step (S403, S413) is detected by said gas quantity detecting means, and an airtightness judging step (S405, S406, S407, S408, S415, S416, S417, S418) in which it is judged that there is an airtight failure in said working-gas circulating type gas engine (S408, S418) when the absolute value of a quantity deviation of circulation gas (|ΔP|, S405, S415) which is the difference between said predetermined quantity 1 (Ps) and said circulating gas increase (Pd) is a predetermined quantity 2 or more (S406: No, S416: No), while it is judged that there is no airtight failure in said working-gas circulating type gas engine (S407, S417) when the absolute values of said quantity deviation of circulation gas (|ΔP|) is less than said predetermined quantity 2 (S406: Yes, S416: Yes).