A method of wastewater reutilization by which reusable water can be stably obtained from a wastewater. Ozone is added to a wastewater, such as water resulting from sewage treatment, in such a small amount as to result in a residual ozone concentration as measured before membrane filtration of 0.01-1.0 mg/L. Ozone is thus brought into contact with fine solids contained in the wastewater to alter the surface properties of the fine solids so that the solids are easily to coagulate. Thereafter, a coagulant, e.g., PACl, is added from a coagulant addition device (3). The fine solids are coagulated in a coagulation tank (5) or a line mixer and the resultant water is subjected to membrane filtration with an ozone-resistant separation membrane (6) such as ceramic membrane. Thus, reusable water is obtained which has a residual ozone concentration, as measured after filtration through the membrane, less than 0.5 mg/L.