In order to more assuredly secure a space in a pericardial cavity in accordance with a change in the visual field range of an endoscope, an endoscope adapter (2) according to the present invention is provided with: a protruding part (2c) which is attached to an endoscope (1) having a bent section (1b) in the vicinity of the distal end and which protrudes, at one portion in the circumferential direction of the endoscope (1), from a position on the proximal end side relative to the bent section (1b), in a longitudinal axial direction in which the endoscope (1) extends in a state where the bent section (1b) is not bent; and a guide through hole (2d) which is formed in such a manner as to penetrate the protruding part (2c) in the protruding direction thereof and through which a thin and long guide member (3) for guiding the endoscope (1) to a desired observation site can pass in the protruding direction.
