PURPOSE: To test the heat stability of lubricating oil under high-temperature conditions by providing a seal part which comes into linear contact with the plane part of a test piece and a cooler which prevents the temperature of a sample from rising to a prescribed temperature or more.

CONSTITUTION: A plate type test piece 4 is placed on a test piece fitting table 1 which has a collar with a sharp edge 2 and a plane type electric heater 6 is placed thereupon. Then, a heat insulator 7 is stacked, an upper lid 3 with a clamping screw is clamped, and a heat insulator 8 is further covered thereon. A cooling water jacket 9 is provided integrally at the upper periphery of an oil tank A. Then the electric heater 6 is powered on to hold the test piece at the prescribed temperature and then the sample oil is splashed to below the test piece for a constant time by using a splash rod fitted to a splash rod fitting opening 2. The heater is powered off and cooling is carried out; and the weight of a load sticking on the test-piece is measured. Consequently, a heat stability test of the lubricating oil is conducted under high temperature conditions of 350W650°C.