PURPOSE: To confirm a fluid leak from the primary side of a valve box to a secondary side, not requiring another part like a short pipe and reduce an installation cost and save a space by installing an open/close device for opening/ closing this secondary side for an outside in the secondary side of the valve box.

CONSTITUTION: When it is required that a piping 2 is further installed increasingly from the end of an existing piping 1, a small open/close valve 19 is opened before removing a close flange 8. Then, when the interception state of a valve device 14 is incomplete, as the fluid such as high pressure gas or liquid leaked to the secondary side 17 of the valve box 15 is discharged from the small open/ close valve 19 to the outside, the interception state of the valve device 14 is confirmed. When the small open/close valve 19 is opened and also the valve device 14 is opened before the increase of the piping 2, a blocked rubbish is discharged outside the valve box 15 through the small open/close valve 19. Therefore, a fluid leak from the primary side 16 of the valve box 15 to its secondary side 17 can be confirmed, not requiring another part like a short pipe having a connection flange and a small valve and an installation cost can be reduced and a space saving can be achieved.