PURPOSE: To readily form a thin film of amorphous polymolybdic acid and to obtain a radiation-sensitive material using the thin film by dissolving metal Mo in an aqueous solution of hydrogen peroxide to give an aqueous solution of polymolybdic acid, applying the solution and drying.

CONSTITUTION: Metal Mo is dissolved in an aqueous solution of hydrogen peroxide to give an aqueous solution of polymolybdic acid. The polymolybdic acid is shown by the general formula MoO3.xH2O2.yH2O(x is 0-0.4; y is 1-2) and has peroxo structure. Then the aqueous solution of polymolybdic acid is applied and dried to give a coating film of amorphous polymolybdic acid. The thin film provided as a colored film on a substrate, and opposing electrode facing the thin film and an electrolyte retained between both the films are processed into a radiation-sensitive material.