PURPOSE: To make the torque at the time of starting and stoppage small and the pumping action of a power generating groove uniform by making the end part of a shaft bearing space between a thrust shaft bearing surface and a thrust bearing surface wider than the central part, and the end part deeper than the central part in the power generating groove.

CONSTITUTION: Since the shaft bearing space between a thrust shaft bearing surface 5 and a thrust bearing surface is formed so that the space dimension δ2 of the end part is larger than the space dimension δ1 of the central part, only the central part contacts in the state of stoppage. Therefore, only the contact resistance of the central part whose circumferential speed is small is concerned at the time of starting and stoppage, and the torque at the time of starting and stoppage is small. On the other hand, since a dynamic pressure generating groove 6 is made so that the groove depth is most shallow in the central part and deeper as goes to the end part, the deeper the groove depth is, the larger the generated load capacity is, and nearly the same load capacity as the central part is obtained at the end part where a shaft bearing space 3 is wide, the inflow and retention of lubricating oil in the shaft bearing space 3 become satisfactory, and the shaft bearing performance is enhanced.