PURPOSE: To surely monitor leakage from the seam of a coating material by forming an inspection hole through a pressure casing and the coating material so that the hole can communicate with an inspection passage formed so as to cover the seam with a covering strip piece on the inside of the coating material.

CONSTITUTION: In order to monitor the sealability of the abutted seam 4 of a coating material 1 during the operation of a pressure vessel, a connection nipple 12 having an inspection hole 12a which is opened below an inserting strip piece 9 is fitted to pressure casing layer 2 and pressure casing pipes 11a-11d. For fitting the nipple 12, a hole 13 is bored through the pipes 11a-11d and the hole 13 is filled up by build up welding 14. Then, after the nipple 12 is welded to the weld 14 by forming a circumferential seam 15, the inspection hole 12a is extended to the strip piece 9 through the weld 14 and a filling strip piece 8. When the nipple 12 is connected to a suitable monitoring device, the sealability of the abutted seam 4 can be monitored during the operation of the pressure vessel.