An overtube is provided with a flexible elongated portion provided with a channel, the channel is configured to pass therethrough in a longitudinal direction of the elongated portion so as to insert a medical device; a shape fixing mechanism provided in the elongated portion, and the shape fixing mechanism switches the state of the elongated portion between a bendable state and a shape-fixed state; and a gate mechanism disposed on a base-end side of the elongated portion, the gate mechanism is configured to close the channel when the state of the elongated portion is switched to the bendable state, and open the channel when the state of the elongated portion is switched to the shape-fixed state.