this invention provides a polyamide separation membrane , prepared from high molecular weight polyamide having a repeating unit represented by the formula and a process for separating gaseous mixtures using them . ## str1 ## wherein , r is an aliphatic or aromatic organic group derived from dicarboxylic acid or halide . the polyamide separation membrane according to the present invention has a large free volume and good permeation properties such as permeability and permeation selectivity .