An over-tube of the present invention is an over-tube which guides a device having an insertion section to be inserted into a body of a patient, when inserting the insertion section into or retracting the insertion section from the body of the patient, wherein: a gas-supplying passage is defined between an inner face of the over-tube and the device to be inserted into the over-tube； the gas-supplying passage communicates with a gas-supplying port which is provided on a proximal end side of the over-tube, and a communicating port which is formed on an over-tube insertion section and supplies a gas into a trachea of the patient； and a first sealing member which secures an air passage for communicating the gas-supplying passage and the trachea of the patient via the communicating port by sealing between a body wall of the patient and the communicating port, is provided on a periphery of an over-tube insertion section.本發明的一種外套管，其當一裝置具有可插入於病人體內之一插入段部插入於病人體內或自病人體內撤出時，導引該裝置，其中：一氣體供應通道，其界定於該外套管的一內表面與要插入於該外套管內的裝置之間；該氣體供應通道是與設置於該外套管的一近端側上之一氣體供應口連通，及與形成於一外套管插入段部上且將氣體供應於病人氣管內之一連通口連通；及一第一密封構件，其設置於一外套管插入段部的環周上，以藉由密封於病人體壁與該連通口之間，穩固一空氣通道，用以經由該連通口連通該氣體供應通道與病人的氣管。
