An ultraviolet (UV) irradiation apparatus according to an embodiment includes a treatment tank, a UV irradiation member, a UV sensor, and an air outlet unit. The treatment tank includes a water supply opening to supply therethrough treated water to be further treated and a water drainage opening to drain therethrough the treated water. The UV irradiation member is provided inside the treatment tank and irradiates treated water with UV light when the treated water passes through inside of the treatment tank. The UV sensor is provided inside the treatment tank and measures a dose of UV irradiation from the UV irradiation member. The air outlet unit is connected to an air outlet hole provided at a position higher than a horizontal plane that passes through the UV sensor, and is provided to release, to the outside of the treatment tank, through the air outlet hole, air that accumulates inside the treatment tank when the treated water passes through the inside of the treatment tank.