PURPOSE: To improve the permselectivity of gas, in a separation membrane comprising a polyquinazolone polymer, by introducing a polysiloxane structure into the main chain of the aforementioned polymer.

CONSTITUTION: A separation membrane comprises a substantially linear polyquinazolone polymer having a repeating unit represented by formula I [wherein R1 is a tetravalent aromatic group, R2s are respectively independently alkyl or an aromatic group, R3 is (P+2)-valent hydrocarbon or a heterocyclic group, Z is carboxyl, sulfone or metal salts thereof and P is independently 0 or an integer of 1W4 at every unit] and a repeating unit represented by formula II[wherein R1 and R2 are same to the aforementioned groups, R4s are respectively independently monovalent alkyl or an aromatic group and (m) is an integer of 2W4]. This membrane has the excellent properties of the polyquinazolone polymer and has further improved gas permeability.