PURPOSE: To provide a method which, as a posttreatment for the preparation of a product, can lower the viscosity of a highly dielectric cellulose deriv. such as carboxymethylcyanoethyl cellulose in a simple manner with a high efficiency.

CONSTITUTION: One material selected from among ferrous sulfate, cobalt oxide, and alumina and one material selected from among tetrasodium ethylenediaminetetraacetate, disodium ethylenediaminetetraacetate, nitrilotriacetic acid, trans-cyclohexanediaminetetraacetic acid, diethylenetriaminepentaacetaic acid, ethylene glycol bisetherdiaminetetraacetic acid, and triethylenetetraminehexaacetic acid are dissolved or dispersed in an aq. soln. of an org. solvent having a suitable water content. A powdery, highly dielectric cellulose deriv. is wetted with the solution or the dispersion, and an aq. org. solvent soln. having a suitable water content and contg. hydrogen peroxide is added thereto, followed by depolymerization with heating.