Cutting tool

This cutting tool (1) is provided with: a tool body (2) that rotates around an axis (O); a cylindrical part (7) provided at the tip on the distal side in the axis (O) direction of the tool body (2) and extending in the axis (O) direction; a boring tool (3) describing the shape of a shaft, detachably fitted on the radial-direction inner side of the cylindrical part (7) and extending so as to project from the cylindrical part (7) toward the distal side; and a head part (9) describing a cylindrical shape, fitted on the radial-direction outer side of the cylindrical part (7), and provided on the outer peripheral section thereof with a cutting blade. The peripheral wall of the cylindrical part (7) is provided with an outer wall (19), an inner wall (20), and a fluid chamber (18) defined between the outer wall (19) and the inner wall (20), the interior of the fluid chamber being filled with fluid. The pressure of the fluid in the fluid chamber (18) is increased, whereby the outer wall (19) elastically deforms toward