Downhole plug, especially designed for closing an annular conduit in an oil or gas well, under high pressure and temperature. It has two radially expandable ring elements 13, 15, which are arranged on a carrying cylindrical element 32, between two mutually axially movable pressure elements 12, 14, having an expansion sleeve in between. The ring elements can, by insertion of the downhole plug in a well, be expanded from an inner position, seen radially, to a sealing position against the wall of the well. The radially expandable ring elements 13, 15, comprises a closed series of circumferentially overlapping seal elements 13A, 15 A. By the expanding movement they are mutually moved in the circumferential direction, thereby maintaining a seal against each other. They form an outer sealing surface 38, which can provide a seal against a cylindrical pipe wall, and they have an inwards facing sealing surface 47.