To provide an ultraviolet irradiation system which easily enables a check of an illuminance sensor.

An ultraviolet irradiation apparatus 1 provided on a flow path passing a liquid to be treated comprises an ultraviolet lamp 6 irradiating the liquid to be treated passing the flow path with ultraviolet light to treat the liquid to be treated and the illuminance sensor 7 for monitoring the illuminance of the ultraviolet lamp. An illuminance sensor check device 3 comprises an inspecting ultraviolet lamp 29, an inspecting reference illuminometer 31, a sensor installation part 30 for setting the illuminance sensor to be an inspection target and a comparison device for checking the reliability of the illuminance sensor to be the inspection object by comparing the output of the reference illuminometer and the output of the illuminance sensor to be the inspection object set on the sensor installation part. The illuminance sensor 7 is checked by the illuminance sensor check facilities 3 by removing the illuminance sensor 7 from the ultraviolet irradiation apparatus 1 to set it on the sensor installation part 30.