An Occluder for occluding a passage in a circulatory system comprises at least one occluding body (4), for occluding the passage, an expanding unit (1) comprising at least one fixation structure for fixing the occluding body (4,5) within the passage, wherein at least a part of the expanding unit (1) is expanding into said fixation structure thereby expanding the occluding body and a locking unit (2,3) for locking the expanding unit (1) in its expanded state. The expanding unit (1) comprises a first and a second end and wherein the first end is attached to a first element of the locking unit (2,3) and the second end is attached to a second element of the locking unit (2,3), the first and second elements of the locking unit (2,3) being brought in locking engagement with each other when the occluding body (4,5) is expanded, wherein the occluder further comprises a jacket(6,7) which envelops at least a part of the expanding unit (1). This occluder ensures that no parts of the occluder can wander within the human or animal body and enhances the growth of the body tissue over the occluder.
