A pericardial-cavity observing method including: a step of inserting an endoscope sheath and an endoscope into a space between the heart and the pericardium; a step of disposing a protruding portion closer to the pericardium than an optical member is so that an angle that is formed between a centerline that passes through a center of the protruding portion and a center of the optical member and a tangent of the pericardium that passes through a foot of a perpendicular line drawn, from the center of the optical member of the endoscope, to the pericardium sagging down from the protruding portion toward the heart becomes greater than an angle formed between the centerline and an external common tangent of the protruding portion and the optical member; and a step of observing the heart by means of the endoscope.