The present invention relates to a tube of a surgical instrument for a colonoscope, the tube having an improved structure of the surgical instrument for the colonoscope, inserted through an insertion tube of the colonoscope, so as to recognize that the surgical instrument arrives near the tip of the colonoscope, recognize the location of the surgical instrument inside the colonoscope, and measure the size of observed lesions. The tube of a surgical instrument for a colonoscope comprises: a tube line having a terminal end advancing into the insertion tube of the colonoscope through a working channel provided in the colonoscope inserted into the body so as to examine intestines, and a starting end connected to an adjuster of the colonoscope outside the working channel a sensing part including a core wire body having a core wire, which is made from a thin steel wire and inserted into a guide hole (210) inside the tube line, and of which a snare part at one side thereof is exposed to the terminal end of the tube line and of which the other side is connected to a handle part of the adjuster, and having sensing points formed on the tube line, inserted into the insertion tube of the colonoscope, at the position corresponding to the length of the insertion tube and a measuring part marked with measurement gradations at the terminal end of the tube line so as to measure the size of a polyp in a state in which the measuring part passes through and is exposed at the insertion tube of the colonoscope, wherein the core wire is exposed at the terminal end of the tube line and the snare part further has markers distinctively distinguished from the measuring part of the tube line. Since the tube line insertion length is recognized by the color of the sensing part of the tube line inserted into the working channel, the locations of the tube line and a snare in a surgery environment can be accurately recognized, thereby simultaneously protecting patients and preventing a mistake of u
