PURPOSE: To grasp reliability in a short time by exposing a ceramic sintered body containing tetragonal zirconia under pressurization and heating of a pressure exceeding 1 atmospheric pressure and a specific temperature range within water or steam atmosphere for a specified amount of time.

CONSTITUTION: For testing durability of a ceramic sintered body, zirconia containing tetragonal zirconia is exposed to pressurization/heating of a pressure exceeding 1 atmospheric pressure and a temperature range of 100-1250°C within water or steam atmosphere whose water content exceeds 0.6g/l for a specified amount of time. It is desirable that water content within water or steam should be equal to or more than 2.5g/l. It is desirable that pressure is equal to or more than 5 atmospheric pressure. Since treatment time is short, temperature should be 150-500°C. An auto crave device 1 which is used for test has a stainless vacuum container 2, a thermocouple 3, a test piece 4, water 5, a heating heater 6, and a manometer. According to the testing method, reliability of the ceramic sintered body containing tetragonal zirconia can be grasped in a short time.