Even if ozone and hydrogen peroxide solution are simply mixed with water to be treated to perform oxidative destruction of harmful substances, such as dioxins, PCB and the like, it is difficult to decompose the harmful substances, with desired decomposing effects. It is an object of the present invention to provide an advanced water treatment process and system which are based on ozone treatment capable of providing more reliable treatment effects. To attain the above object, according to the present invention, ozone treatment is carried out in which minute bubbles of ozone having an average particle diameter of 0.5 to 3 mu m are brought into contact with water to be treated, which contains the harmful substances, such as dioxins and PCB. By carrying out a desired combination of the ozone treatment as an essential treatment, and a hydrogen peroxide treatment, an ultraviolet radiation treatment, an electrolysis treatment, and a carbonized filter medium treatment, it is possible to realize enhanced effectiveness of the water treatment.