The invention concerns a membrane filtration type hollow fiber. It is composed of a single material phase which material is a chain high polymer and capable of forming fibers by the spinning of the high polymer. The cylindrical wall defining the hollow fiber represents a three-dimensional net-like structure of fine filtering passages of 0.05 to 1 micron, preferably 0.1 to 1 micron. This structure occupies substantially the whole mass of the tubular wall or a closed ring band zone when seen in any cross-section of the wall, providing thus an effective and active filtering zone. The pore rate amounts to at least 55%, and active points of the filtering passages for governing minimum passable sizes of the passages for solids included in a filtering liquor to be treated upon are distributed at random throughout at least the active zone. These sizes are nearly uniform.