A light source device of a projector adapted to irradiate blue light, red light, green light and yellow light in turn on an image-forming element and to form a color image on a screen by the image-forming element, comprising: a light source section adapted to generate the blue light; a rotatable light path-switching disk which is disposed in a light path of the blue light and is adapted to periodically switch the light path of the blue light between a first light path and a second light path. a rotatable phosphor member disposed in the first light path and including a first phosphor (5a', 5a") adapted to generate green light or red light through irradiation of the blue light emitted from the light source section and a second phosphor adapted to generate yellow light through the irradiation of the blue light emitted from the light source section; a dichroic mirror configured to combine light in the first light path with light in the second light path and direct the combined light to the image forming element; a rotatable color-changing disk disposed between the dichroic mirror and the image forming element and adapted to select from the combined light any one of: the blue light emitted from the light source section, or the green light emitted either from the first phosphor (5a', 5a") or from the second phosphor, or the red light emitted either from the first phosphor (5a', 5a") or from the second phosphor, or the yellow light emitted from the second phosphor, and to irradiate the selected light to the image-forming element; wherein the rotatable light path-switching disk and the rotatable color-changing disk rotate in synchronization at the same rotational speed. a light source section adapted to generate the blue light; a rotatable light path-switching disk which is disposed in a light path of the blue light and is adapted to periodically switch the light path of the blue light between a first light path and a second light path. a rotatable phosphor member disposed in the first light path and including a first phosphor (5a', 5a") adapted to generate green light or red light through irradiation of the blue light emitted from the light source section and a second phosphor adapted to generate yellow light through the irradiation of the blue light emitted from the light source section; a dichroic mirror configured to combine light in the first light path with light in the second light path and direct the combined light to the image forming element; a rotatable color-changing disk disposed between the dichroic mirror and the image forming element and adapted to select from the combined light any one of: the blue light emitted from the light source section, or the green light emitted either from the first phosphor (5a', 5a") or from the second phosphor, or the red light emitted either from the first phosphor (5a', 5a") or from the second phosphor, or the yellow light emitted from the second phosphor, and to irradiate the selected light to the image-forming element; the blue light emitted from the light source section, or the green light emitted either from the first phosphor (5a', 5a") or from the second phosphor, or the red light emitted either from the first phosphor (5a', 5a") or from the second phosphor, or the yellow light emitted from the second phosphor, and to irradiate the selected light to the image-forming element; wherein the rotatable light path-switching disk and the rotatable color-changing disk rotate in synchronization at the same rotational speed.