The invention has for its object to provide a preparation method for preparing an anion-exchangeable LDH by decarbonation of a carbonate ion type LDH, which makes sure de carbonation is implemented with safety in a continuous manner while crystal shape, crystal structure and crystallinity are kept intact.The invention provides a preparation method for preparing an anion-exchangeable, layered double hydroxide wherein a carbonate ion type layered double hydroxide (LDH) having a composition represented by a general formula: QxR(OH)z(CO32−)0.5-y/2(X−)y.nH2O where x is indicative of a numeral range of 1.8≦x≦4.2; z is indicative of 2(x+1); y is indicative of a minimum value of at least 0 that increases to less than 1 when anions (X−) remain or a part of anions is introduced; Q is a divalent metal ion; R is a trivalent metal ion; and n is 2±2 is used as a starting material, and y in said general formula increases to a maximum of 1 by substitution of a minus monovalent anion (X−1) at a carbonate ion site thereby implementing substitution, characterized in that the starting material is dispersed in an aqueous solution mixed with a salt containing minus monovalent anions (X−) in an amount enough for substitution at the carbonate ion site while said aqueous solution is kept at a pH (hydrogen ion exponent) of greater than 4 to less than 7.