A tube cutting device for cutting a tube of a waste steam generator, the tube cutting device comprising: a body configured to transmit power; a tube-shaped rotary body provided on a side surface of the body and having a horizontal rotary shaft configured to rotate with the power supplied from the body; a plurality of clamps provided along an outer peripheral surface of a first side end portion of the rotary body and configured to fix the rotary body to the tube to be cut; a saw provided on opposite second end portion opposite to the first side end portion of the rotary body and configured to rotate together with the rotary body to cut the tube, wherein the saw is formed in a doughnut shape and comprises a hollow portion formed on a center portion thereof and a plurality of saw blades forming the saw; and a fitting member having a conical shape and disposed on an opposite side to a side of the saw facing the body, and wherein a peak portion of the fitting member is configured to be inserted into the hollow portion formed on the center portion of the saw; and wherein the body comprises: a ball screw configured to drive the fitting member forwardly and backwardly along a horizontal line toward and from the hollow portion of the saw to move the plurality of the saw blades in a direction of an outside of the tube from an inside of the tube, and a saw blade driving motor configured to move the plurality of saw blades toward an inner peripheral wall of the tube. a body configured to transmit power; a tube-shaped rotary body provided on a side surface of the body and having a horizontal rotary shaft configured to rotate with the power supplied from the body; a plurality of clamps provided along an outer peripheral surface of a first side end portion of the rotary body and configured to fix the rotary body to the tube to be cut; a saw provided on opposite second end portion opposite to the first side end portion of the rotary body and configured to rotate together with the rotary body to cut the tube, wherein the saw is formed in a doughnut shape and comprises a hollow portion formed on a center portion thereof and a plurality of saw blades forming the saw; and a fitting member having a conical shape and disposed on an opposite side to a side of the saw facing the body, and wherein a peak portion of the fitting member is configured to be inserted into the hollow portion formed on the center portion of the saw; and wherein the body comprises: a ball screw configured to drive the fitting member forwardly and backwardly along a horizontal line toward and from the hollow portion of the saw to move the plurality of the saw blades in a direction of an outside of the tube from an inside of the tube, and a saw blade driving motor configured to move the plurality of saw blades toward an inner peripheral wall of the tube. a ball screw configured to drive the fitting member forwardly and backwardly along a horizontal line toward and from the hollow portion of the saw to move the plurality of the saw blades in a direction of an outside of the tube from an inside of the tube, and a saw blade driving motor configured to move the plurality of saw blades toward an inner peripheral wall of the tube.