To stabilise the precipitated quantity of maganese dioxide by accurately adjusting the concentration of respective components in permanganic acid solution composed of permanganate and pH adjusting agent in the case of oxidation treatment.

In the case of oxidation treatment of an insulated board B with permanganic acid solution L in a reaction vessel B, a level sensor 2, photoelectric sensor 3 and pH sensor 4 respectively detect the liquid level of the permanganic acid solution L, the oxidation treatment area of insulated board P and the pH of the solution L with the passage of time and output these detected values to a solution component managing part 1. Based on these detected values, the solution component control part 1 detects the decrease in change of moisture from the solution L caused by evaporation, successively supplies water so as to fix the total quantity of solution in the reaction vessel B, simultaneously detects the decrease in permanganate concentration and the increase in pH with the oxidation treatment and supplies the permanganate and pH adjusting agent so as to fix respective components.