A densely wound coil and a wire which passes through the coil are inserted into a flexible part which has a flexibility. The coil which is fixed in the vicinity of a distal end of the wire has a proximal end thereof which is regulated in movement toward a rearward side by a stopper. The wire which extends rearward from the proximal end of the coil has a proximal end thereof which is drawn or pulled toward the rearward side by angular movement of an adjustment knob which is provided in the vicinity of a forward end of an operation part. By this traction, a compressive force is relatively applied to the coil to make flexibility of the coil changeable. Indicia associated with the adjustment knob alerts the user to the level of flexibility to which the adjustment knob is moved.