The gastric ring has two ends (11, 12) and, between these ends, an intermediate portion (13) of solid section for coming to bear simply against a stomach portion that is to be surrounded. The first end (11) forms an opening (115) in which the second end (12) can be inserted. The second end (12) is suitable for being prevented from moving in said opening (115) in at least two distinct positions, thereby enabling the inside diameter (d'.sub.1) of the ring to be adjusted to its implantation conditions.