An illumination optical apparatus and a projection type display apparatus capable of enhancing an illumination efficiency; having an ellipsoidal mirror, a light source arranged at a first focal point of the ellipsoidal mirror, a stop arranged closer to said light source side by a predetermined distance from a second focal point of said ellipsoidal mirror, and a positive refractive collimator lens for collimating a light flux emitted from the stop to a parallel light flux. A condition (0.01