A packer device for sealing against an inner surface of a surrounding pipe or casing string, preferably for isolating zones or sections in an oil well, said packer device comprising - a tubing body and - at least one sealing element (7a,b), and further comprising - a cylinder/piston arrangement that comprises a cylinder that is closed at one end and having an axially movable piston arrangement at the other end, and said cylinder having a closed volume containing a fluid, such as a gas or liquid, arranged to expand when exposed to heat thereby expanding said closed volume and thereby exerting a mechanical pressure on movable elements (11, 12), initially locked in a fixed position by means of at least one shear member, and where the movable elements (11, 12) are adapted to be released into an operative state at a predetermined axial force from said mechanical pressure exerted by the expanding closed volume in the cylinder, resulting in that the at least one sealing element (7a,b) is pressed radially outwardly in such a way that it seals the packer device against the surrounding pipe-/casing string, wherein the movable elements comprise an internal piston that is arranged inside the cylinder and which is axially movable along the tubing body, characterised in that, - the cylinder is axially movable along the tubing body, - the movable elements further comprise an external piston that is connected to the internal piston and axially movable along the tubing body, - it comprises two expandable sealing elements (7a, 7b) attached to the tubing body and positioned one on each side of the cylinder/piston arrangement and against respective stop elements (15a, 15b), - the cylinder and the external piston are configured to move apart when the movable elements (11, 12) are released into the operative state and the closed volume is expanded, and thereby expanding the two sealing elements (7a, 7b) by means of the external piston being conically shaped and the cylinder having a conically shaped outer end, the packer device comprises a locking system that is integrated in the cylinder and the external piston and which locking system keeps the sealing elements expanded. - a tubing body and - at least one sealing element (7a,b), and further comprising - a cylinder/piston arrangement that comprises a cylinder that is closed at one end and having an axially movable piston arrangement at the other end, and said cylinder having a closed volume containing a fluid, such as a gas or liquid, arranged to expand when exposed to heat thereby expanding said closed volume and thereby exerting a mechanical pressure on movable elements (11, 12), initially locked in a fixed position by means of at least one shear member, and where the movable elements (11, 12) are adapted to be released into an operative state at a predetermined axial force from said mechanical pressure exerted by the expanding closed volume in the cylinder, resulting in that the at least one sealing element (7a,b) is pressed radially outwardly in such a way that it seals the packer device against the surrounding pipe-/casing string, - the cylinder is axially movable along the tubing body, - the movable elements further comprise an external piston that is connected to the internal piston and axially movable along the tubing body, - it comprises two expandable sealing elements (7a, 7b) attached to the tubing body and positioned one on each side of the cylinder/piston arrangement and against respective stop elements (15a, 15b), - the cylinder and the external piston are configured to move apart when the movable elements (11, 12) are released into the operative state and the closed volume is expanded, and thereby expanding the two sealing elements (7a, 7b) by means of the external piston being conically shaped and the cylinder having a conically shaped outer end,