PROBLEM TO BE SOLVED: To provide an overtube of a structure which can hold an internal treatment instrument inserted in the body from a tube body at the tube body while maintaining an opening hole of a human body to be an expanded state by the tube body.SOLUTION: A tube body 110 is inserted in the inside of a human body HB from an opening hole OH. An internal balloon 120 is extended to the outside of the tube body 110 inserted from the opening hole OH in the inside of the human body HB. An external holding member 130 is located in the outside of the human body HB which counters the opening hole OH at the outside of tube body 110. A tool holding mechanism 140 holds the internal treatment instrument inserted in the tube body 110 freely attachably and detachably.
