In an extreme ultraviolet (“EUV”) light source apparatus, uneven angle distribution images of EUV light are detected prior to an adjustment function by a detector, and angle distribution image data is recorded. Movement data corresponding to the recorded angle distribution image data is also recorded. The movement data corresponds to a movement amount and direction that the optical focusing means is moved from a position in which the angle distribution is even to the position in which the corresponding uneven angle distribution image is obtained. For the adjustment, a current angle distribution property image is detected by the detector and is compared with the uneven angle distribution property image data stored, and image data which is most closely matched with the current angle distribution property is selected. The movement data that corresponds to the selected image data is read out, and the light focusing optical means is moved based thereon.