A working fluid injection apparatus for injecting a working fluid into a gap between a housing and a shaft of a bearing unit of a fluid dynamic pressure bearing. The apparatus has an adapter configured to support the bearing unit in a state in which an end portion of the shaft protrudes from an open portion of the housing. A cover member has an upper opening, a lower opening, and a tapered inner surface with a radial dimension that increases gradually from the lower opening to the upper opening. The cover member is configured to be mounted in contact with an upper surface of the housing when the bearing unit is supported by the adapter so that the lower opening of the cover member surrounds the open portion of the housing and so that the protruding end portion of the shaft and the tapered inner surface of the cover member form a reservoir portion that communicates with the open portion of the housing and that is configured to store a working fluid.