The present invention relates to a flex-knife that enables a technique such as tissue excision to be carried out more easily, and is provided with: a rod-shaped high frequency knife which is endoscopically inserted into a body cavity and is for performing cutting procedures; a wire, the high frequency knife connected to the front end thereof; a sheath consisting of an insulating material, into which the wire is inserted; a main body, the back end of the sheath being connected thereto; and a slider to which the back end of the wire is fixed and which is disposed to the main body to enable sliding in the axial direction; wherein the high frequency knife has a large diameter part at its distal end, the large diameter part having an outer diameter that is larger than the inner diameter of the sheath.