An illumination optical device is provided with a light source device and a uniform illumination optical system. To a light flux emission side of an expanded portion of the light source lamp of the light source device, which is opposite to a reflector of the illumination optical device, a reflection member is provided. An effective reflection plane diameter of an elliptical reflector of the light source device is 30 mm or more but 50 mm or less, Furthermore, assuming that a first focal distance of the elliptical reflector is f1, and a second focal distance thereof is f2, f1 becomes equal to or more than 5 mm, but equal to or less than 50 mm, and f2/f1 becomes in a range from 4 to 7 inclusive. With such a structure, it becomes possible to provide an illumination optical device and a projector with which a solution can be provided to achieve further system downsizing, reducing or preventing light use efficiency from lowering, keeping a light quantity at a predetermined level or more, and reducing or preventing a manufacturing cost increase.