The expansion instrument includes an outer and inner circumference walls.The outer circumference wall is configured to be deformable in directionswhere adiameter of the outer circumference wall expands and shrinks. Theinnercircumference wall is located in a position surrounded by the outercircumference wall,and configured to be deformable in the directions where a diameter of theinnercircumference wall expands and shrinks. Between the outer circumference wallandthe inner circumference wall, an internal space where fluid can be introducedisprovided. When the fluid is not introduced into the internal space, the outerand innercircumference walls are deformed in the direction where each diameter shrinks.When the amount of the fluid exceeding the initial volume that the internalspaceinherently has is introduced into the internal space, the outer and innercircumferencewalls are deformed in the direction where each diameter expands.
