To simplify processes, and reduce investment cost in plant construction by supplying hydrogenated organic working solution, oxygen-containing gas, and water extracting solvent to one reactor for conducting oxidation and extraction in the same space.

Hydrogenated working solution goes from a first tubular reactor 1 to a first phase separator 2, passes a second tubular reactor 3 and a second phase separator 4, and passes a third tubular reactor 5 and a third phase separator 6 to be oxidation working solution, and it is hydrogenated to be hydrogenated working solution to be circulated. Pure water is supplied through the phase separator 6 corresponding to the third tubular reactor 5, it passes through the second tubular reactor 3 to get into the second phase separator 4, and it passes the first tubular reactor 1 to get out of the first phase separator 2 as a product of hydrogen peroxide solution. Oxygen-containing gas is supplied to each tubuar reactor 1, 3 and 5. Each phase separator 2, 4 and 6 comprises two continuous phase separators, so gas is separated from liquid in one of them, and that two sorts of liquid phases are separated in the other. The inside of each tubular reactor 1, 3, 5 is strongly agitated.