PURPOSE: To carry out the titled treatment with the effective use of ozone, by oxidizing and removing NO2 ion in the secondary disposed night soil water by an electrolysis method, and then oxidizing the treated water with ozone to decolor the water.

CONSTITUTION: Secondary disposed night soil water 1 introduced and homogenized in a reservoir 2 is introduced into an electrolysis device 7 of a type such that an electrolyte is carried through a narrow space at a high speed. In the device 7, the water is electrolyzed by direct current from a power source 8. In this case, anodes 7a in the device 7 are made of insoluble material. Nascent oxygen is generated on the surfaces of the anodes 7a by the electrolysis of water, and used for the oxidation of NO2 ion in water to NO3 ion. Thereafter, the treated water is supplied into an ozone aerating tower 3, where the water is brought in contact with ozonized oxygen or ozonized air generated by an ozone generator 5, so that coloring components in the water are decomposed by oxidation and decolored, and highly purified water is discharged into a reservoir 6.