A rotating-anode X-ray tube has a rotary pipe rotatably supported in a casing, and a target which is fixed to one end of the rotary pipe and can rotate along with the rotary pipe, the target being cooled by a flow of cooling fluid. A cooling-fluid sealing device is provided between the casing and the rotary pipe and is a mechanical seal. The mechanical seal has a rotary ring which is rotatable along with the rotary pipe and axially movable, a stationary ring fixed to the casing, and pressing means for pressing the rotary ring against the stationary ring. The rotary and the stationary rings come into surface contact with each other within a plane perpendicular to an axis of rotation. The rotary ring is made of carbon and the stationary ring is made of silicon carbide ceramics, both materials being electrically conductive.