PURPOSE: To improve the operativity by letting each cable passing through a hermetic wall penetrate an independent cylindrical sleeve, and performing confirmation of the leak from a hermetic seal part for each cylindrical sleeve.

CONSTITUTION: A plurality of through cables 1 pass through a hermetic wall. A hermetic seal part 6 has the cable 1 at its center. A fastening metal 8 of the cable 1 is installed between the cable 1 and a leak confirmation sleeve 7 which is welded to an end plate 5, so that the axial movement of the cable 1 is restricted. A leak confirmation tool 11 is mounted outside the sleeve 7. The pressure is sealed into a hermetic chamber 14 of the sleeve 7 by a pressurized gas through a pressurizing device 12 for each cable 1. In this case, since the volume of the hermetic chamber 14 is small, the leak can be confirmed with a relatively low pressure without being influenced by the change of the ambient temperature. If the leak from the seal part 6 in the hermetic chamber 14 is confirmed, the tool 11 is detached and a mending seal material 15 is added and filled from an end part of the sleeve 7. Then, the confirmation of a leak is carried out again.