According to an embodiment, an ultraviolet (UV) irradiation apparatus includes a treatment tank, a UV irradiation member, a UV sensor, and an air outlet unit. 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 waterlasses through the inside of the treatment tank.