Abstract: An overtube for an endoscope includes: a tube body having a main lumen through which the endoscope is inserted and an air-supply lumen through which gas flows; a fixing balloon provided on an outer peripheral surface of a distal end of the tube body, expandable outward from the outer peripheral surface and contractible toward the outer peripheral surface; an air supply device configured to send the gas to the air-supply lumen; and an airtight valve unit having a tubular portion communicating with the main lumen at a rear end of the tube body, the airtight valve unit closing the gap between the endoscope inserted through the tubular portion into the main lumen and an inner peripheral surface of the tubular portion.