Method of preparing high purity light gas by multiple-step gas separation

A method for preparing a high purity light gas consisting of a member selected from the group consisting of hydrogen and helium, comprising subjecting a feed gas containing at least 90 molar % of a light gas consisting of a member selected from the group consisting of hydrogen and helium and substantially no carbon dioxide to a gas-separating procedure in 2 to 5 steps through 2 to 5 gas-separating membrane modules, each of said gas-separating membrane modules comprising at least one aromatic imide-polymer gas-separating membrane and having a gas permeating rate ratio P.sub.A /P.sub.CH4 of 100 or more, wherein P.sub.A represents a permeating rate of hydrogen or helium and P.sub.CH4 represents a permeating rate of methane gas, in such a manner that a fraction of said feed gas, which has permeated through and has been delivered from a preceding gas-separating membrane module, is fed to a next gas-separating membrane module without increasing the pressure of the delivered fraction of the feed gas.