Abstract: The present invention provides a method for detection of a vacuum leak of a vacuum vessel provided in an equipment, for example, an absorption chiller, and in which water and a solution (for example, lithium bromide solution) are charged. The gas in the vacuum vessel is purged by an auxiliary air pump while spraying helium as trace gas by a helium spray gun to the outer surface of the vacuum vessel under inspection of the absorption chiller, a part of the purged gas is passed through a dehumidifier where the steam is removed from the gas, a part of the purged gas is depressurized by reducing the partial pressure of the steam to zero, and it is detected by a helium-leak detector whether or not the trace gas (helium) leaks from the vacuum vessel. This method and an apparatus for carrying out the method permit to check for any vacuum leak and quantitatively determine it quickly, easily and with a high sensitivity whether the equipment under inspection is in or out of operation.