(EN) Provided are a light source device, an endoscope system, and a method of operating a light source device, which are capable of suppressing the occurrence of an abnormal image accompanying the mode switching at the time of mode switching. The light source section (20) emits light in a plurality of wavelength bands. The light source section (20) can change the light emission ratio of light in each wavelength band. When switching between the first illumination light having the first light emission ratio and the second illumination light having the second light emission ratio different from the first light emission ratio, the first illumination light is switched at least in one frame or more during the switching period. The switching period illumination light having a switching period light emission ratio different from the second light emission ratio and the second light emission ratio is emitted.
