A light source device comprising: a light source section which generates any one of blue light, red light, and green light; a phosphor which generates a fluorescence including the two colors other than the color of the light emitted from the light source section; a color-changing section which changes one of the two colors of the fluorescence emitted from the phosphor to another color regularly and irradiates it to the image-forming element; and a light path-switching section which switches a light path in which a fluorescence excited by the color light emitted from the light source section passes towards the color-changing section and a light path in which the color light emitted from the light source section passes towards the image-forming element regularly.