PURPOSE: To obtain a soundness monitoring device for the structure in an atomic reactor by arranging a test piece comprising the same material quality as that of the in-pile structure in the similar environment as that of the in-pile structure, and monitoring the soundness of the structure in the reactor based on the confirmation of the breakdown of the test piece.

CONSTITUTION: A cylindrical tube 2 has the same material quality as that of a structure to be monitored. A shape memory alloy 6 imparts stress on the tube 2. Electrodes 8a and 8b, which are electrically insulated from the tube 2, are provided in the space in the tube 2. These parts are provided in a test piece 1. This device comprises the test piece 1, a processing device, which processes the electric signals from the electrodes 8a and 8b of the test piece 1, and a display device, which displays the result of processing in the processing device. The test piece 1 comprising the same material quality as that of the in-pile structure in the reactor is arranged in the same environment as that of the in-pile structure. The soundness of the in-pile structure is monitored based on the confirmation of the breakdown of the test piece 1. Thus, the soundness- monitoring device for the in-pile structure in the atomic reactor is obtained.