There is provided a method for preparation of porous glass films, preferably in the form of a hollow filament, comprising heating glass films, formed from sodium borosilicate glass having a composition which can be phase-separated by heat-treatment into a soft phase easily eluted with a hot acid solution and a hard phase not easily eluted with the same solution, to separate the glass phase into the two phases and then subjecting the glass film to plasma etching in an atmosphere containing gaseous fluorine-containing compound(s) before or after acid elution of the soft phase. The thus obtained porous glass films have a uniform pore diameter and smooth surfaces and can be used as a variety of separating membranes.