A pore diffusion type flat membrane separation apparatus X including a plurality of flat membranes 7 and a plurality of flat plate-like supports 1 arranged alternately with each other, each flat membrane 7 defining a plurality of pores and configured to separate a predetermined dispersed substance contained in a solution by a pore diffusion technique, each flat plate-like support 1 having a flow conduit 2 on one or both faces thereof. A ratio between a spatial volume of the flow conduit 2 and a membrane area of the flat membrane 7 is set from 0.04 to 0.4 cm. The flat plate-like support 1 includes, in at least two positions in a lateral face thereof water conduits 3 in communication with the flow conduit 2, so that flow directions of the solution in the flow conduits 2 of upper and lower flat plate-like supports 1 across the flat membrane 7 may be substantially same directions. The flat plate-like support 1 and the flat membrane 7 can be assembled with and disassembled from each other.