Abstract: A magnetic fluid sealing device basically includes a housing; a rotating shaft rotatably supported in said housing by bearings; and a magnet and magnetic pole pieces, which are arranged in such a manner as to encircle said rotating shaft with micro gaps put therebetween. A magnetic field formed by magnet forms magnetic fluid films in the micro gaps to thereby magnetically seal high pressure/vacuum regions positioned at both ends of the rotating shaft. The sealing device is characterized in that the housing, and a mounting cap for fixing the magnetic fluid sealing portion and the bearings at specific positions in the housing are made of a magnetic material, and the materials of the housing and bearing and the dimension of the mounting cap are changed, to thereby reduce a leaked magnetic flux.