average particle diameter of the alumina crystal particles measured for the above inverted electronic image by the planimetric technique was 20 u03bcm further the material cut out from the interior of the base material center portion was subjected to icp light emission analysis to measure the amount of yag the result was 0 weight the sample pieces of 10 u00d7 10 mm square were cut out from the above sintered body and their breakage toughness were measured by the if method using a vickers indenter the result was 4 1 mpa m u00bd which is approximately equal to the breakage toughness of the sintered body of alumina the three point bending strength measured for these samples was 360 mpa the sample pieces with the one side being the surface of the sintered body itself having a thickness of 2 mm and an area of 10 u00d7 10 mm square were cut out from the above sintered body and were subjected to a plasma exposure test under the same condition as that in the first embodiment the result of etching rate nm hr was 1 or less further ten sample pieces with the one side being the surface of the sintered body itself having a thickness of 2 mm and an area of 20 u00d7 20 mm square were cut out from the above sintered body these samples were subjected to the heat cycle test under the same condition as in the first embodiment before the number of the heat cycles reaches 100 times the local separation of the yag layer was confirmed for three of the ten samples incidentally the ceramics material having the same property as described above can be acquired as follows a molding or calcined body having a composition system of the alumina.