[Problem to be solved] To kill bacteria with ultraviolet light that can effectively kill bacteria with ultraviolet light and to suppress bacteria increase due to photoreactivation after disinfection.[Means to solve the problem] Bacteria in water 9 exposed outdoors are effectively killed with ultraviolet (UV) light by suppressing post-treatment increase in the bacteria population due to photoreactivation. The apparatus shines UV light on the water 9 to kill bacteria and has UV light emitting diodes (LEDs) 1 that emit UVA light with a primary emission peak of 320nm-400nm. The antibacterial action of the UVA light emitted by the UV LEDs 1 prevents proliferation of bacteria in the disinfected water 9 due to photoreactivation.
