A gastrointestinal-tract constricting method includes: while observing the gastrointestinal tract by inserting an endoscope into the gastrointestinal tract, forming a spreading block that suppresses 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 to a gap between the spreading block and the mucosa basal layer while forming a contact surface between the spreading block and the substance along the circumferential direction of the gastrointestinal tract.