A high-frequency incision instrument for an endoscope is provided with an electrically insulative flexible sheath constituted of a distal tube disposed at a tip portion thereof and a proximal tube disposed on a proximal side such that one is loosely inserted into the other so as to rotate around an axial line thereof, a high-frequency electrode exposed along a lateral surface of the distal tube and connected to a conductive operating wire extending throughout inside the distal tube and the proximal tube, so that a high-frequency current is supplied to the high-frequency electrode through the operating wire, and so that rotating the operating wire at a proximal end portion of the proximal tube around an axial line causes the distal tube to rotate around the axial line with respect to the proximal tube, thus to change an orientation of the high-frequency electrode.