PURPOSE: To accurately measure the equilibrium adsorbing amount of a multicomponent gaseous mixture by forming a circulation pipeline by further providing a separate connection pipe between a variable capacity cell and an adsorbing cell and providing a circulation pump on the way of the pipeline.

CONSTITUTION: The second connection pipe 13 is provided between a variable capacity cell 5 and an adsorbing cell 2 and a circulation pipeline is formed between the cells 2, 5 by the first and second connection pipes 4, 13 and a circulation pump 15 is arranged on the way of the circulation pipeline. Since this circulation pipeline constitutes almost all of parts within an entire measuring system, when the pump 15 is operates in such la state that the measuring system is filled with gas, the gas is circulates through almost all of parts within the measuring system of flow under stirring/mixing and, therefore, the composition gradient and temp. gradient of the gas within the entire measuring system are certainly eliminated and, as a result, the composition of the gas becomes uniform and the temp. of the gas is easily controlled to set temp. to be capable of being kept constant.