A light source device for use in an image capturing device such as an endoscope capable of reducing size and cost, and preventing heat generation at an illuminated position. An illumination unit of the endoscope includes LED light sources for emitting ultraviolet light, fluorescent fibers for generating red, green and blue light respectively by irradiation of the ultraviolet light, a sampling light source, and an excitation light source. Upon capturing a normal image, the ultraviolet light is emitted sequentially from the LED light sources to cause the fluorescent fibers to generate the red, green and blue light. The light of each color is passed through a light guide and irradiated on an observation area of a living body. Thereafter, reflected images attributable to the light of each color are captured with a CCD image capturing element of a charge multiplying type, and captured images are displayed on a monitor.