PROBLEM TO BE SOLVED: To provide a tubular space cutoff method and a device, capable of surely cutting off water by a simple and compact water cut-off device, and easily applicable, for example, even when a tubular space is a cross-sectional square shape except for a cross-sectional circular shape. SOLUTION: A flexible airtight annular hose 3 is arranged along the inner periphery of the tubular space 1. A watertight large diameter flexible tube 6 is loosely fitted outside the annular hose 3 along a lower half part of the tubular space 1. A water cut-off wall 7 is formed for shielding a range surrounded by the tube 6. The tube 6 is brought into pressure contact with an inner wall of the tubular space 1 by inflating the annular hose 3 by fluid pressure. The lower half part of the tubular space 1 is cut off by the water cut-off wall 7. COPYRIGHT: (C)2008,JPO&INPIT