An insertion opening attachment is used in combination with an endoscope apparatus configured to observe the inside of a subject. The attachment is attached to an insertion opening opened in the subject into which an insertion unit of the endoscope apparatus is inserted, wherein the insertion opening attachment comprises an attachment main body having a flanged cylindrical shape which is fixed to a peripheral edge region of the insertion opening in a state the insertion opening attachment is inserted in the insertion opening. The attachment main body comprises is attachment state detection unit which detects whether the attachment main body is attached to the insertion opening and an insertion state detection unit which detects whether the insertion unit is inserted in the insertion opening.
