PURPOSE: To effectively utilize exhaust ozone to efficiently perform ozone treatment by bringing exhaust ozone discharged in a water treatment process, wherein ozone is utilized, into contact with slurry produced in said process.

CONSTITUTION: Exhaust ozone discharged in a water treatment process 3, wherein ozone is utilized, is brought into contact with slurry produced in said process, e.g. chemical precipitation process 2, sand filtration process 5, and activated carbon filtration process 6, in a exhaust ozone contact process 8. As a result, harmful substances in the slurry, e.g. chlorine organic compound, color, smell, trihalomethane precursor, anionic surface-active agent, bacteria, ionic or colloidal Fe or Mn, etc., are decomposed and oxidized and at the same time the exhaust ozone is treated. That is, the exhaust ozone is effectively utilized and treatment of ozone is efficiently carried out.