Abstract: A mechanical shaft seal for sealing the space between a stationary element and a rotating element in order to separate an inner pressure medium space from an external space, preferably the atmosphere. The seal comprises a sealing ring mounted on one of the elements and a counter ring mounted on the other of the elements. The sealing ring and the counter-ring are provided with annular sealing slide surfaces pressed against each other by means of the pressure of the pressure medium. An intermediate space for a sealing fluid is limited by one of the elements, the sealing ring and said slide surfaces. An annular piston is provided on said sealing ring in order to press the sealing ring in a direction closing said slide surfaces and thereby to increase the closing force subjected by the sealing fluid on said slide surfaces.