A working fluid injection apparatus has an upper chamber that stores a working fluid and a lower chamber that removably receives a bearing unit having a housing and a shaft that is inserted in the housing to form between the shaft and the housing a gap into which the working fluid is to be injected. A partition divides the upper chamber and the lower chamber into top and bottom parts, respectively. A first exhaust system brings the inside of the upper chamber to a decompressed state. A second exhaust system brings the inside of the lower chamber to a decompressed state. A valve unit opens and closes an internal cavity of the lower chamber relative to an external environment. A tapered hole is formed in the partition and communicates the upper and lower chambers to one another. A tapered shaft is inserted into the tapered hole from the upper chamber. A shaft drive unit moves the tapered shaft in the axial direction in the tapered hole.