PURPOSE: To easily detect the defect of a tube by sealing gaseous nitrogen under pressure to a shell side and measuring the oxygen concn. in the tube.

CONSTITUTION: The gaseous nitrogen is introduced under the pressure from a gaseous nitrogen blow port 1 to the shell 4 side of a cooler until the internal pressure thereof attains about 0.3kg/cm2. On the other hand, a rubber or wooden plug 7 is provided on one end of the tube 3 and a probe 5 for measurement is inserted to the other open end and the oxygen concn. in the tube 3 is measured by an oxygen densitometer 6. Whether the oxygen is diluted or not is discriminated. The generation of a defect in the tube 3 or not is thereby made possible extremely easily and the quick maintenance and repair of the device are made possible.