To provide a method for quickly quantifying mercury contained in a hardly soluble ceramic with high sensitivity, without losing the mercury at the decomposition time.

After weighing the hardly soluble ceramic sample into a decomposition vessel, sulfuric acid and phosphoric acid are added thereto, and the vessel is plugged, stored in a pressure vessel, and heated, to thereby decompose the sample; and mercury concentration in the solution acquired thereby is measured. Alternatively, after weighing the hardly soluble ceramic sample into the decomposition vessel, sulfuric acid and phosphoric acid are added thereto, and the vessel is plugged, stored in the pressure vessel, and irradiated with a microwave, to thereby decompose the sample; and mercury concentration in the solution acquired thereby is measured.