PURPOSE: To decrease the rotating speed to start floating in the thrust direction and shorten the time to attain the stationary flotation quantity by providing a pressure regulating valve on a flow hole communicating a pressure chamber formed between the end face of a shaft and the thrust bearing face to the outside of a housing.

CONSTITUTION: A flow hole 11 is provided at the center of the thrust bearing bottom 3 of a housing 1, and a pressure regulating ball 12 is brought into contact with it. When the housing 1 is rotated, the housing 1 is floated by the action of the dynamic pressure generating groove 5 of a shaft 4. During the low-speed rotation, the ball 4 is kept in contact with the flow hole 11, the fluid does not flow out, and the housing 1 is floated at a lower rotating speed. When the rotating speed is increased and the dynamic pressure is made larger than the energizing force of the ball 12, the fluid flows out, and the flotation quantity of the housing 1 becomes a fixed value. Therefore, the rotating speed to start floating is decreased and the time to attain the stationary flotation quantity is shortened by regulating the pre-load by the ball 12.