PURPOSE: To open pores in the surface which is made hydrophilic, of a membrane precursor by stretching this membrane precursor which is a hydrophobic thermoplastic polymer and at least one surface of which has been subjected to a treatment for making hydrophilic.

CONSTITUTION: The membrane precursor which is formed by subjecting at least one surface of the molten membrane precursor formed by melt-extrusion molding of the hydrophobic thermoplastic polymer, such as polyolefin or polypropylene, to the treatment for making it hydrophilic, then solidifying the membrane precursor, or the membrane precursor which is formed by subjecting at least one surface of the solid membrane precursor formed by melt-extrusion molding of the thermoplastic polymer to the treatment for making it hydrophilic is used. The porous membrane having the pores communicating the front and rear of the membrane, or a heterogeneous membrane formed with the pores which are open on the front surface which is made hydrophilic of the membrane but are not communicated with the other surface, etc., are obtd. when the pores are generated in the membrane precursor by stretching. Then, this membrane is useful in the case liquid exists on one side of the membrane and gas is in contact with the other side.