To provide a cutting tool that can remove a thimble tube from a fuel assembly in a short time, without requiring skill and can be easily applied to MOX fuel.

The cutting tool 10 of the thimble tube comprises a positioning section 12 to be inserted into the thimble tube 74 fixed to a sleeve tube 73 of an uppermost support lattice, a retaining section 11 to be retained by an operator 90 to fix the positioning section 12 after the movement and positioning, a cutting edge 13 that is mounted inside the positioning section 12 and can be widened and made to come out from the inside of the positioning section 12 to the outside in the radial direction, and a cutting edge driving section 14 that, while the positioning section 12 is fixed, widens and outputs the cutting edge 13 from the inside of the positioning section 12 to the outside in the radial direction under a cutting operation of the operator 90, and cuts the thimble tube 74 from the inside to the outside, together with the sleeve tube 73 fixing the thimble tube 74 under a cutting operation by the operator 90.