Stress relaxation is an important property for spring materials. In particular it is desired to know the relaxation at an elevated temperature from practical viewpoint, however standard method for the measurement of the property is not yet established. Authors made a new apparatus for testing the high temperature stress relaxation of thin plate materials. A thin sheet of Cu-Ni-Sn alloy, which has an excellent stress relaxation character at room temperature, is submitted to inspection for the testing in the temperature range at 423K. The measurement gave a reliable result as like as measurements obtained from conventional methods at room temperature.