The object of the invention is to provide a porous composite substrate with excellent durability, which can be physically or chemically stable and can be used in severe environment, by providing a coating layer with high corrosion resistance and insulating properties or good electric conductivity if required, on a porous base material having open pores and a filtration function, and to use this porous composite substrate effectively as filters and field electrodes for water treatment or the like. The present invention is a porous composite substrate comprising a composite member, the composite member comprising a porous base material and a diamond layer which covers the porous base material, wherein the porous base material and the composite member have open pores to serve a filtration function. The porous base material is an insulator, a semiconductor, or a metal.