Abstract: In a vacuum laser irradiating apparatus disposed between a laser generator and a vacuum vessel in which an object to be irradiated with a laser beam is placed, an outer window plate is airtightly fixed to the outer opening of the apparatus body to maintain the internal cavity of the apparatus body under a vacuum and to pass the laser beam therethrough. This outer window plate is reinforced so as not to be distorted due to pressure difference between an inside vacuum of the apparatus body and an outside atmospheric pressure. This is because the outer window plate is made of an expensive pure monocrystalline material and therefore it is preferably to reduce the thickness thereof for material cost reduction.