A medical device insertion method includes: inserting an endoscope into an overtube until a distal end of the endoscope protrudes from a distal end of the overtube; grasping a part of the endoscope protruding from the distal end of the overtube and inserting the endoscope into the large intestine until the distal end of the endoscope is between the splenic flexure and the sigmoid colon of the large intestine; and inserting the overtube together with the endoscope into the large intestine until the distal end of the endoscope is beyond the splenic flexure while grasping the overtube from an outer radial direction so as to deform the overtube in an inner radial direction until an inner surface of the overtube is pressed against an outer circumferential surface of the endoscope, and then inserting the overtube into the large intestine until a reinforced region is placed in the splenic flexure.