PURPOSE: To obtain a lightweight porous filter medium excellent in ability to remove Mn contained in water and having low filtration resistance by subjecting a fine particle-shaped mixture consisting of acid clay, alumlna, etc., to specified treatments and imparting a prescribed apparent specific gravity after firing.

CONSTITUTION: A fine particle-shaped mixture consisting of acid clay, alumina, kibushi clay, kaolin and hard zeolite and having about 200μm average particle diameter is slurried, molded, dried and sintered at a temp. at which molten glass is not formed in correspondence with the blending ratio among the starting materials, preferably at <1,000°C. After cooling and crushing, classification and selection are carried out and the resultant powder is immersed in a KMnO4 soln. until a certain quantity of the soln. is adsorbed. The powder is then dried and fired at a temp. corresponding to the ratio among the starting materials to attain an apparent specific gravity of 0.85-0.90.