An engaging mechanism, which restricts rotation of a cutter section about an axis of a sheath, is provided. In accordance with an advancing/retreating operation of an operation wire by an operation slider, switching is made between a rotation restriction position where rotational restriction of the cutter section is effected by the engaging mechanism and a restriction release position where the cutter section is released from the rotation restriction position and is made rotatable about the axis of the sheath relative to the sheath.