PURPOSE: To provide a separation membrane keeping excellent gas permeability and having further improved separation property, constituted by combining a vinyl triorganosilane polymer with a 1-monoalkyldimethylsilylpropine polymer having a specific formula.

CONSTITUTION: A gas permeable membrane consists of 1-monoalkyldimethylsilylpropine polymer A having a repeating unit represented by formula I (wherein R1 is 1W12C straight chain or branched chain alkyl) and a vinyl triorganosilane polymer B. As the polymer B, a vinyl triorganosilane polymer represented by formula II (wherein R2 is H, alkyl, cycloalkyl or aryl and R3 and R4 are alkyl, cycloalkyl or aryl) is designated. The coefficient of O2-permeation of this separation membrane is usually about 10-9W10-7cc.cm/cm2.sec.cmHg and the coefficient of separation of O2 and N2 thereof is usually within a range of about 2.0W5.0.