A gastrointestinal-tract constricting method includes, while observing the gastrointestinal tract by inserting an endoscope into the gastrointestinal tract, forming a spreading block that blocks infiltration of a substance, which damages a mucosa basal layer of the gastrointestinal tract, into the muscular layer underlying the mucosa basal layer, the spreading block being formed along a circumferential direction of the gastrointestinal tract and between the mucosa basal layer and the muscular layer; and supplying the substance along the circumferential direction of the gastrointestinal tract and to a surface of the mucosa that lies within a region that overlaps an inner side of the gastrointestinal tract with respect to the spreading block out of the entire circumference of the gastrointestinal tract in a transverse section of the gastrointestinal tract.