A water treatment apparatus dissolving an oxidizer in wastewater at a fast speed and at a high efficiency to decompose substances at a high decomposition rate. The apparatus includes an ultraviolet radiation part having an elongated tubular configuration, and receiving an axially extending ultraviolet radiation unit with a space between an inner peripheral surface of the ultraviolet radiation unit and the ultraviolet radiation unit to pass wastewater; and an oxidizer mixing part disposed adjacent to and upstream of the ultraviolet radiation part for mixing an oxidizer with the wastewater. The ultraviolet radiation unit irradiates the wastewater with ultraviolet light to decompose the substances to be treated in the wastewater.