PURPOSE: To effectively restore the decomposition capacity of hydrogen peroxide contained in activated carbon in decomposing hydrogen peroxide by bringing water containing hydrogen peroxide into contact with an alkali agent before bringing the same into contact with activated carbon to decompose hydrogen peroxide.

CONSTITUTION: When hydrogen peroxide contained in semiconductor washing waste water if decomposed and removed, water containing hydrogen peroxide is passed through an activated carbon(AC) column 1 from piping 11 to obtain treated water from piping 12. An alkali aq. soln. such as sodium hydroxide is passed through the AC column 1 to regenerate activated carbon. At the time of regeneration, at first, the supply of raw water is stopped and a pump P1 is operated to pass the alkali aq. soln. in an alkali water tank 2 through the AC column 1 and regeneration waste water is discharged from piping 15. After the washing of the AC column 1 is completed, the pump P1 is stopped while a pump P2 is operated and backwashing water in a backwashing water tank 3 is,,, passed through the AC column 1 to wash away alkali.