PURPOSE: To improve heat resistance by heat-treating reaction liquid containing the suspension of clay, the salt of one or more kinds of a cation M and a compound (AB) for supplying OH- ions by thermal hydrolysis and then separating a product.

CONSTITUTION: The 20 g/l suspension of the clay of montmorillonite or the like is prepared, NaCl is added, Ca cation is substituted by Na cation, then washing is performed and Cl- ion is removed. The salt of the cation M hydrolyzed by pH3.0-10.0 such as Al(NO3)3 or the like is added to the clay suspension for 0.1-6.0 by weight to the clay by oxide conversion. Then, the aqueous solution of the compound (AB) such as urea and cyanamide, etc., is prepared, an amount for turning a molar ratio with the cation to 0.5-3.0 is added and the reaction liquid is heated to 80-120°C for 1-10 hours. The processed suspension is filtered by a glass filter, separated and then washed, the product is dried and then calcined at 300-800°C and bridged clay is obtained.