PURPOSE: To enable the treatment of various materials such as separation, concentration, diffusion to be easily carried out by inserting a porous tube, the one end of which is sealed, into an internal pressure type tubular permeable membrane to increase the amount of permeated liquid.

CONSTITUTION: A porous tube 2 is inserted into an internal pressure type tubular permeable membrane 1, the outer diameter of the tube 2 being smaller than the inner diameter of the tubular membrane 1. The porous tube 2 is fixed to the membrane 1 in such a manner that the center line of the tube 2 is coincided with that of the membrane 1. Liquid to be treated is supplied from a liquid-inflow part A having an opening end 5 to a concentrated liquid discharge part B having a sealed end 4. The liquid is passed through a part corresponding to the outer side of the porous tube 2 or simultaneously through the inner side and the outer side of the tube 2, thereby the amount of permeated liquid is increased, and the reduction in the amount of the permeated liquid in course of time due to the soil on the membrane surface of the tubular membrane can almost be prevented.