Activated regular-shaped clay particles forming macropores therein, preserving fine structure of the dioctahedral smectite clay minerals, and having particle diameters controlled to lie within a range in which they exhibit excellent filtering property. The invention further provides method of producing activated regular-shaped clay particles comprising the steps of adjusting the particle size of the dioctahedral smectite clay minerals to possess a volume-based median diameter (D 50 ) of from 1 to 10 µ m as found by a laser diffraction method, mixing an acid-soluble or acid-decomposing inorganic compound to the milled product thereof, granulating the mixture into regular particles having a volume-based median diameter (D 50 ) of from 10 to 60 µ m as found by a laser diffraction method, and treating the regular-shaped particles with an acid while maintaining the particle shapes.