PURPOSE: To leave a metal catalyst existing in pores of a sinter and to clean the sinter until gas formation ceases by treating the sinter used for filtration of metal catalyst suspension with an aqueous solution of hydrogen peroxide. CONSTITUTION: In a cleaning apparatus of a first metal filter of hydrogen peroxide plant, a cover 2 of a filter unit having a sinter 3 is disposed between an upper part 4 and a lower part 5 of a cleaning tank 1. Upon cleaning an aqueous solution of hydrogen peroxide is conducted from a container 6 along the tube 16. Thus the aqueous solution of hydrogen peroxide flows from the upper part 4 into the sinter 3, that is, to their so called clean side and through the sinter 3 into the lower part 5, that is, to the so called dirty side. That is, the solution of hydrogen peroxide flows in the opposite direction of the direction in which a metal catalyst suspension flows during the filtering of the metal catalyst in the catalytic hydrogenation of anthraquinone or the like and releases the metal catalyst caught in the pores of the sinter 3. And ceasing of gas formation indicates that no significant quantities of metal catalysts are left.