PURPOSE: To perform filtration with good filtration efficiency and good separating efficiency by making a precise filtration using a filter medium precoated with a inorganic solid particles which is difficulty soluble in water and with a water- soluble high molecular compound.

CONSTITUTION: A porous base material of an average pore diameter of 0.01W10 microns is precoated with a mixture of a inorganic solid particles which is difficultly soluble in water with average grain size of 1 micron or less, e.g., calcium carbonate, silica, etc., and 0.1wt% or less a water-soluble high molecular compound of an average molecular weight of 50,000 or more, e.g., polyvinyl alcohol, etc., under a pressure of 10kg/cm2 or less. The filtration of fine substances with the precoated base material permits filtration and separation with good filtration efficiency and also enables the precoat layer to be removed by peeling off after filtration.