To enable removal of a hazardous substance such as residual ozone (or residual hydrogen peroxide) and an oxidized by-product cheaper than a conventional method and apparatus in a water treatment method and apparatus by ozone treatment (or accelerated oxidation treatment).

The water treatment method comprises the steps of: bringing ozone gas into contact with water to be treated in an ozone contact tank 2 and mixing it to carry out ozone treatment; and then in a hazardous substance removal tank 5, removing the hazardous substance comprising the residual ozone dissolved in treated water after the ozone treatment and the oxidized by-product produced in accompany with the ozone treatment. The hazardous substance removal tank 5 includes at least a deozonized material filling layer 6 having the function of decomposing dissolved ozone and a biological carrier layer 7 carrying a microbe for decomposing the oxidized by-product, and the residual ozone and the oxidized by-product are removed by passing the treated water after the ozone treatment into the hazardous substance removal tank 5.

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