PURPOSE: To function at all times in response to change in load and rotating speed by forming one of a pair of opposite members capable of rotating relatively of a porous ceramic sintered body imprignated with magnetic fluid, and providing a magnetic field forming means controlling flow of magnetic fluid.

CONSTITUTION: When a load applied to a rotary shaft 7 exceeds a load capacity of gas bearing, current supply to a solenoid coil is controlled to form a magnetic field in a specified direction between respective magnets 17, thereby magnetizing magnetic fluid L existing in the magnetic field to be a positive value. The magnetic fluid L between the ceramic plate 10 and the lower opposite member 3 moves to permeate in the ceramic plate 10. Then the fluid L impregnated in the ceramic plate 10 oozes out on the upper surface side of the ceramic plate 10 so that the part between the ceramic plate 10 and the upper opposite member 8 is filled with the fluid L. In this way, the working fluid between them is change over to the fluid L from air, thereby a large load can be supported and the rotary shaft 7 is supported in the positive direction with the dynamic pressure of magnetic fluid L. By inverting the conduction direction, returning to the original condition is made.