A gastrointestinal tract constricting method according to the present invention aims to constrict the gastrointestinal tract by contracting a desired region of the gastrointestinal tract by a simple and low-invasive procedure. The method includes forming spreading blocks while observing the gastrointestinal tract with an endoscope inserted into the gastrointestinal tract, the spreading blocks being formed at a position between a mucosa layer and a muscle layer and on both sides of a target region, which is to be damaged by a substance, in a circumferential direction of the gastrointestinal tract so that the spreading blocks block spreading of the substance in the circumferential direction of the gastrointestinal tract to prevent spreading of the substance to an outer side of the target region, and supplying the substance to a mucosal surface of the target region or to the position between the mucosa layer and the muscle layer after formation of the spreading blocks.