A gas separation membrane with excellent gas separation properties. The gas separation membrane of the present invention is substantially free from pinholes and consists essentially of a crosslinked polyolefin or a crosslinked polyarylene oxide. The gas separation membrane of the present invention is produced by evaporating a solution containing a polyolefin or a polyarylene oxide having an active functional group which can form crosslinking therebetween or which can react with a crosslinking agent to form a crosslinking.