PURPOSE: To turn the micro bores inside the membrane and also the inner face of membrane hydrophilic and durable by carrying out the two-stage plasma treatment to treat the membrane in low temperature plasma with the gas which able to form the hydrophilic radical after the micro porous hollow yarn membrane being treated with the plasma polymerizable gas.

CONSTITUTION: The micro porous hollow yarn membrane 3 made of hydrophobic polymer such as polyethylene and the like is placed in the belljar-type plasma treater 1, and is treated with the plasma polymerizable gas such as methane gas having gas pressure 10-4W1torr under the high-frequency electric discharge. After the said gas is exhausted, the activated gas, which is able to form the hydrophilic radical such as O2 gas, is treated in the low temperature plasma having the activated gas pressure 10-4W1torr. The porous hollow yarn membrane treated with the two-stage plasma turns hydrophilic not only in its outer face but also in the micro bores inside the membrane and the entire inside of the membrane.