According to one embodiment, an ultraviolet irradiation system that sterilizes, disinfects, and inactivates raw water such as city water or underground water, includes a plurality of ultraviolet irradiation devices connected in series, and a controller configured to control output and non-output of the ultraviolet irradiation devices. The ultraviolet irradiation device includes a water processing vessel through which the raw water flows, an ultraviolet lamp configured to irradiate the raw water in the water processing vessel with ultraviolet rays, and an ultraviolet sensor configured to measure an amount of ultraviolet rays emitted from the ultraviolet lamp.