PURPOSE: To prevent the overflow of a magnetic fluid by providing a sink for magnetic fluid to a magnetic path which supplies lines of magnetic force to the magnetic fluid or covering said magnetic path with a material having a high magnetic conductive factor or a material having high permeability.

CONSTITUTION: A magnetic disk device contains magnetic paths 3 and 5 forming a microgap 8 as well as a rotary shaft 1. Then sinks 10 for magnetic fluid are provided to magnetic paths 3 and 5. The shaft 1 is made of a magnetic material, and a magnetic field is formed at the gap 8 by a permanent magnet 4. In this case, magnetic fluids 6 and 7 stay to some extent at sinks 10 and never flow any more. Then the reduction is prevented for both fluids 6 and 7 by filling even the sinks 10 with the magnetic fluid in an early stage of filling.