PURPOSE: To prevent the crack of a sintered body and to improve the yield of a product by heating a non-light transparent zirconia-base sintered body to a heat treatment temp. in an inert gaseous atmosphere, then heat-treating the same in an oxygen atmosphere.

CONSTITUTION: The zirconia-base sintered body reduced by a hot isostatic press (HIP) treatment of an inert gaseous pressure is previously formed. The non-light transparent sintered body is then heated gradually from room temp. up to the oxidation heat treatment temp. in the inert gaseous atmosphere of Ar, etc. After the sintered body is heated up to a prescribed temp., the introduction of oxygen into the furnace is started and the progression of the oxidation is uniformized to obtain the light transparent sintered body. The unequal progression of the oxidation is thereby obviated and the crack to be generated at the time of the oxidation is prevented. The yield of the product is thereby improved.