The sheath includes a protruding section that protrudes from a ring-shaped distal end surface of a sheath body and a lifting part that lifts the protruding section within the pericardial cavity to a position separated from the surface of the heart toward the pericardium. The protruding section includes a pressing surface disposed toward the pericardium when the protruding section is lifted by the lifting part, and also includes an opening that is provided at a position facing the pressing surface in a radial direction and at which a distal end surface of the endoscope is capable of appearing in the radial direction as an endoscope bending section operates.