An ultraviolet irradiation apparatus according to an embodiment includes a reaction tank including an inlet port for introducing to-be-treated water, and a discharge port for discharging to-be-treated water after treatment, a first ultraviolet lamp, and an ultraviolet monitor including an ultraviolet sensor. The first ultraviolet lamp is accommodated in a first protection tube including both end portions fixed in the reaction tank. The ultraviolet monitor is accommodated in a second protection tube, the second protection tube is disposed in parallel to the first protection tube, and both end portions of the second protection tube are fixed in the reaction tank.