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 sealing member which surrounds the communicating port and seals between a body wall of the patient and the communicating port, is provided on a periphery of an over-tube insertion section.