PURPOSE: To check a safety of the heat conductive pipe by inspecting a presence or nonexistence of variation of liquid surface by a method wherein a liquid level meter is arranged in an upper header of a heat exchanger, a liquid surface in the upper header is kept at a specified level, the liquid surface is pressurized by nonactive gases and then left as it is for a specified period of time.

CONSTITUTION: First coolant inlet nozzle 2 is arranged at a side part of the outer barrel 7, a first coolant outlet nozzle 3 is arranged at the lower header 18, inner barrel 10 having inlet port 12 and outlet port 13 is arranged inside the outer barrel 17, the upper and lower pipe plates 8, 11 and the heat conductive pipe 9 are mounted in the inner barrel 10, the interior of the upper header 17 is applied as a top chamber 15, a secondary coolant flow pipe 4 is inserted into the lower chamber 16 at its central part, a liquid level metal 14 is installed at the upper header 17, then the outlet 5 is opened therein. A liquid surface level 36 of the secondary coolant is kept at its specified level and a presence or nonexistence of damages in the heat conductive pipe 9 is checked in reference to a presence or nonexistence of a variation of the liquid surface 36.