The purpose of the present invention is to more reliably prevent a load from being applied to surrounding tissue in a body cavity from when a medical device is inserted into a channel in a state whereshape fixation has been released, or from when shape fixation is released in a state where a medical device has been inserted into a channel. This over tube (1) comprises: a flexible elongated section (3) that is provided with a channel (2) that penetrates through the longitudinal direction; a shape fixation mechanism (4) that is provided to the elongated section (3) and that can switch the elongated section (3) between a curve-able state and a fixated-shape state; and a gate mechanism (13) that is disposed on a proximal end side of the elongated section (3) and closes the channel (2) when the elongated section (3) is switched by the shape fixation mechanism (4) to the curve-able state, and opens up the channel (2) when the elongated section (3) is switched by the shape fixation mechanism(4) to the fixated-shape state.