A solute-separating membrane consisting of a polyoxadiazole series resin containing at least one of the structural units of formulae: ##STR1## wherein Ar and Ar' are same or different multivalent aromatic residues, and R is an alkyl, cycloalkyl or aralkyl group having from 1 to 7 carbon atoms, and swollen to a swelling degree of 1.7-15 with a swelling agent of water, a solvent miscible with water, or a mixture of water and the solvent has a high separating ability and further has high mechanical strength and chemical and thermal stabilities.