A method for producing activated clay particles having similar shapes which comprises a step of adjusting the particle size of a dioctahedral type smectite clay mineral to a median diameter (D50) on volume basis of 1 to 10 νm as measured by the laser diffraction method, a step of mixing the resulting adjusted product with an inorganic compound which is soluble in or decomposable with an acid, granulating the resultant mixture into particles having similar shapes and a median diameter (D50) on volume basis of 10 to 60 νm as measured by the laser diffraction method, and a step of subjecting the particles having similar shapes to an acid treatment while maintaining the particle shape and size thereof. The method can be employed for providing activated clay particles having similar shapes which have macro pores therein and retain the fine structure of a dioctahedral type smectite clay mineral, and also have particle diameters controlled within the range wherein they exhibit excellent filtration property.