PURPOSE:To efficiently cool off a main spindle by installing a ring-shaped member made of a permanent magnet in a gap provided around the periphery of the main spindle and filling up said gap with a magnetic fluid. CONSTITUTION:A permanent magnet is installed along the inner periphery of a cylindrical housing 16 of main spindle, and a ring-shaped member 18 magnetized in the thickness direction corresponding to the longitudinal direction of a main spindle 10. This ring-shaped member 18 is installed while maintaining a proper clearance from the main spindle 10, and this space is filled up with a fluid containing powder of iron oxide. On the other hand, at the cylindrical housing of 16 of the main spindle, a path 22 for cooling fluid is located near the outer periphery of the ring-shaped member 18. A plurality of paths 22 are provided in the longitudinal direction of the cylindrical housing 16 of the main spindle. The respective paths are connected together at the tip end portions by a communicating tube 22a, and respective paths and communicating tube function as outgoing and returning paths for the cooling liquid. By doing this, heat exchange between the main spindle and magnetic fluid can efficiently be performed and thus the main spindle can effectively be cooled.