PURPOSE: To make a pressure condition of a pressure vessel unchanged at the time when external force is applied to a test object in the pressure vessel.

CONSTITUTION: A through hole 30a is drilled in a cover 30 which has blocked up the open part of a pressure vessel 29 and external force is applied to a test object 6 through an external force transmission member 32 sliding the through hole 30a. In this case. the external force transmission member 32 is moved, hydraulic oil in which the volume of the vessel 29 at the side of the test object 6 is lessened is transferred through a passage 32e in a cavity part 33 between the external force transmission member 33 and the cover 30. Thereby the external force is applied without changing a pressure condition in the vessel 29 wherein the test object 6 is housed.