Abstract: Provided are an ultraviolet irradiation device, an ultraviolet irradiation method, an illumination device, and an ultraviolet irradiation system capable of efficiently and safely disinfecting an area to be disinfected and capable of preventing reduction in the operating rate of (equipment, etc., in) the area to be disinfected by maintaining the disinfected state. The ultraviolet irradiation device includes an ultraviolet irradiation unit capable of outputting ultraviolet having a predetermined dominant wavelength, and a drive control unit. The drive control unit performs time control of ultraviolet irradiation and non-irradiation by the ultraviolet irradiation unit on the basis of time necessary to disinfect the area S to be disinfected before or during operation and bacterial growth time after the disinfection.