A device (10) for externally applying a local compressive force below the costal arch (R) of a patient for reducing the stomach volume, comprising—an elastic belt (10) that is placeable, under tensile stress, around the upper body of a patient in the area of the costal arch (R), and—a pressure body (24) that is attachable to the elastic belt (12) and designed in such a way that the pressure body exerts a compressive force on the stomach inside the patient according to the tensile stress of the elastic belt (12), wherein the geometry of the pressure body (24) is adapted to the anatomy of the human costal arch and fits snugly therein.