PURPOSE: To shrink a bore diameter without generating a bend in an end part of porous glass tube by heat-treating the porous glass tube while inserting the material of high m.p. into the tube end part thereof and taking off the inserted material.

CONSTITUTION: A porous glass tube 2 for a gas separation module is manufactured by treating a nonporous sodium borosilicate glass. The material 5 such as an alumina rod which has high m.p. and maintains the sufficient strength to hold the glass tube to be heat-treated at high temp. and is not fused with the glass tube is inserted into the inside of tube end part of the glass tube 2 and while the tube end part is exposed to the hydrogen flame 7 kept at 1,100W1,300°C and the glass tube is rotated, it is heat-treated for one minute and the alumina rod 5 is taken off after cooling it.