An insertion opening attachment (11; 41; 51; 61; 71; 81) that is used in combination with an endoscope apparatus configured to observe the inside of a subject and that 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 (11; 41; 51; 61; 71; 81) comprises: an attachment main body (12; 72) having a flanged cylindrical shape which is fixed to a peripheral edge region of the insertion opening in a state the insertion opening attachment (11; 41; 51; 61; 71; 81) is inserted in the insertion opening, and the attachment main body (12; 72) comprises: an attachment state detection unit (16; 42; 62) which detects whether the attachment main body (12; 72) is attached to the insertion opening; wherein the attachment main body (12; 72) furthermore comprises an insertion state detection unit which detects whether the insertion unit of the endoscope apparatus is inserted in the insertion opening, and wherein the attachment main body (12; 72) comprises an insertion state determination unit which determines a positional relationship between the insertion unit and the insertion opening of the object based on an output from the insertion state detection unit and an output from the attachment state detection unit (16; 42; 62), characterised in that said insertion state determination unit determines that the insertion unit is inserted in the insertion opening when the insertion state detection unit detects that the insertion unit is inserted and the attachment state detection unit (16; 42; 62) detects the attachment to the insertion opening. an attachment main body (12; 72) having a flanged cylindrical shape which is fixed to a peripheral edge region of the insertion opening in a state the insertion opening attachment (11; 41; 51; 61; 71; 81) is inserted in the insertion opening, and the attachment main body (12; 72) comprises: an attachment state detection unit (16; 42; 62) which detects whether the attachment main body (12; 72) is attached to the insertion opening; wherein the attachment main body (12; 72) furthermore comprises an insertion state detection unit which detects whether the insertion unit of the endoscope apparatus is inserted in the insertion opening, and wherein the attachment main body (12; 72) comprises an insertion state determination unit which determines a positional relationship between the insertion unit and the insertion opening of the object based on an output from the insertion state detection unit and an output from the attachment state detection unit (16; 42; 62), characterised in that said insertion state determination unit determines that the insertion unit is inserted in the insertion opening when the insertion state detection unit detects that the insertion unit is inserted and the attachment state detection unit (16; 42; 62) detects the attachment to the insertion opening. an attachment state detection unit (16; 42; 62) which detects whether the attachment main body (12; 72) is attached to the insertion opening; wherein the attachment main body (12; 72) furthermore comprises an insertion state detection unit which detects whether the insertion unit of the endoscope apparatus is inserted in the insertion opening, and wherein the attachment main body (12; 72) comprises an insertion state determination unit which determines a positional relationship between the insertion unit and the insertion opening of the object based on an output from the insertion state detection unit and an output from the attachment state detection unit (16; 42; 62), characterised in that said insertion state determination unit determines that the insertion unit is inserted in the insertion opening when the insertion state detection unit detects that the insertion unit is inserted and the attachment state detection unit (16; 42; 62) detects the attachment to the insertion opening.