An apparatus for the treatment of acid reflux disease has an implantable movement restriction device adapted to be at least partly invaginated by a patient's stomach fundus wall. A substantial part of the outer surface of the movement restriction device is adapted to rest against the stomach wall without injuring the latter in a position between the patient's diaphragm and at least a portion of the lower part of the invaginated stomach fundus wall, such that movement of the cardiac notch of the patient's stomach towards the patient's diaphragm is restricted, to thereby prevent the cardia from sliding through the patient's diaphragm opening into the patient's thorax, so as to maintain the supporting pressure against the patient's cardia sphincter muscle exerted from the patient's abdomen. The movement restriction device has a size of at least 125 mm3 and a circumference of at least 15 mm.