PURPOSE: To improve the rheology-controlling function by adding one or more kinds of alkyltrialkoxysiianes represented by specified formulae each in a prescribed amount to bentonite and bonding the alkylsilico groups to the surface of bentonite.

CONSTITUTION: Bentonite is put in deionized water and dispersed with stirring. The obtained suspension is centrifuged and the resultant upper layer composed of a purified bentonite sol is evaporated to dryness so as to obtain anhydraus purified bentonite. 100 pts. wt. purified bentonite is charged into a pot mill and 1 to 10 pts. wt. alkyltrialkoxysilane expressed by formula [RSi(OR1)3] (R is a 1 to 22C saturated alkyl; R1 is methyl, ethyl, propyl or butyl) is added thereto. The resultant mixture is then stirred and crushed in an anhydraus atmosphere so as to bond the alkylsilico groups to the partial surface of the bentonite. The obtained material is dispersed in water, thus obtaining the objective modified bentonite exhibiting a function for controlling rheology of an aqueous fluid. In addition, an organophilic modified bentonite can be obtained by further adding a quaternary ammonium cation thereto.