To more reliably secure the space in the pericardial cavity to comply with the change in viewing area range of an endoscope, an endoscope adaptor is attached to an endoscope having a bending portion near a distal end, the endoscope adaptor including a projecting portion that projects from a position on a proximal end side of the bending portion so as to project in one part in a circumferential direction of the endoscope and in a direction along a longitudinal axis of the endoscope with the bending portion in an un-bent state； and a guide through hole, which is formed in the projecting portion and penetrates through the projecting portion in the projecting direction and through which a long thin guide member for guiding the endoscope to a desired observation site can pass in the projecting direction.
