A gastrointestinal-tract constricting method comprises: while observing the gastrointestinal tract with an endoscope inserted into the gastrointestinal tract, placing a protection substance, which does not damage tissue, in at least one of a region between the mucosal layer and muscular layer of the gastrointestinal tract and a region in a mucosal surface of the mucosal layer so as to form a protection region that protects the mucosa basal layer from a medical substance that damages the tissue； and, after forming the protection region, supplying the medical substance to a mucosal surface in a target region, which is at a position different from the protection region in a circumferential direction of the gastrointestinal tract, or to a position between the mucosal layer and the muscular layer.
