A degassing method of removing a dissolved gas from a liquid and a gas exchange method of exchanging a dissolved gas in a liquid and a gas component in a gas phase include a method using a separation membrane. To provide a separation membrane having solvent resistance while maintaining high gas permeability using poly(4-methyl-1-pentene) excellent in solvent resistance and gas permeability. To achieve the object, there is provided a separation membrane containing poly(4-methyl-1-pentene) as a main component and including a surface layer and an inner layer, at least one surface layer having lamellar crystals, wherein micropores are provided on the surface layer having the lamellar crystals, an opening ratio is 0.1% to 10% when a ratio of the micropores to a membrane surface is taken as the opening ratio and the membrane surface is 100%, and an average pore size of the micropores is 3 nm to 30 nm.