The expansion instrument includes an outer and inner circumference walls. The outer circumference wall is configured to be deformable in directions where a diameter of the outer circumference wall expands and shrinks. The inner circumference wall is located in a position surrounded by the outer circumference wall, and configured to be deformable in the directions where a diameter of the inner circumference wall expands and shrinks. Between the outer circumference wall and the inner circumference wall, an internal space where fluid can be introduced is provided. When the fluid is not introduced into the internal space, the outer and inner circumference walls are deformed in the direction where each diameter shrinks. When the amount of the fluid exceeding the initial volume that the internal space inherently has is introduced into the internal space, the outer and inner circumference walls are deformed in the direction where each diameter expands.
