An endoscopic treatment system 1 of the present invention includes an overtube 2 that is inserted into the body of a patient. An endoscope 7 and a suturing device 8 are inserted into the overtube 2. The overtube 2 has a long and flexible tube main body 3, and a chamber 4 is provided at the distal end of the tube main body 3. The chamber 4 has a lateral hole 10 formed on the lateral surface thereof. The width of the lateral hole 10 is set such that a lesion can be drawn into the lateral hole 10 while preventing other organs on the periphery of the lesion from being drawn into the lateral hole 10.