A medical system includes: an overtube having a first lumen that extends in a longitudinal direction toward a distal end from a proximal end, a second lumen that branches off from the distal end of the first lumen and opens at a distal-end surface, and that is in communication with the first lumen via a first entrance, and a third lumen that opens at an outer circumferential surface and that is in communication with the first lumen via a second entrance； and an attachment that is inserted, together with a medical device, into the first lumen from the proximal-end side, that has a lateral cross-sectional shape making it possible to selectively block at least a portion of either the first or second entrance as a result of the position thereof about a center axis of the first lumen being changed and to guide the medical device to the other entrance.
