A medical instrument can be easily inserted through an overtube including a coupling mechanism for interlocking forward and backward movements of two medical instruments with each other. The coupling mechanism is constituted of a partition wall member 324 that is disposed inside an overtube 300 and extends along a longitudinal axis of the overtube 300 and that has an endoscope guide groove 326 through which an endoscope is inserted, and a treatment tool guide groove 328 through which a treatment tool is inserted； an endosciope fixing tool 430 that is disposed in the endoscope guide groove 326 so as to be movable forward and backward and is coupled with the endoscope； a treatment fixing tool 450 that is disposed in the treatment tool guide groove 328 so as to be movable forward and backward and is coupled with the treatment tool； and a coupling ring 402 that is a ring-shaped driving member which is externally fitted to an outer peripheral part of the partition wall member 324 and is movable forward and backward with respect to the partition wall member 324 along the longitudinal axis and that are engaged with the endoscope fixing tool 430 and the treatment tool fixing tool 450.
