Method for controlling molecular diameters of gases and method for isolating gas

The purpose of the present invention is to change, in accordance with temperature conditions, the molecular diameters of a plurality of gases contained in a gaseous mixture by changing the temperature of the gaseous mixture. From among temperature conditions under which the molecular diameter of a subject gas to be isolated which is contained in the gaseous mixture and the molecular diameter(s) of one or more non-subject gases contained in the gaseous mixture do not overlap, a temperature condition is selected under which there is a porous material having pores with a diameter that is larger than the molecular diameter(s) of either the subject gas or the non-subject gases but is smaller than the molecular diameter(s) of the remainder. The gaseous mixture kept under the selected temperature condition is supplied to a separation means constituted of the porous material, thereby isolating the subject gas from the gaseous mixture via the separation means.