The present invention has an object of improving wear resistance of a thrust bearing portion. A pressure is generated by a dynamic pressure effect of a lubricating oil in a thrust bearing gap C between a thrust bearing surface 11a including dynamic pressure generating grooves and a smooth thrust receiving surface 11b so as to rotatably support a shaft member 2 in an axial direction. The thrust receiving surface 11b is formed as a flat surface, whereas an inclined plane 17 is provided on the thrust bearing surface 11a so as to provide a reduced portion 15 having a decreasing axial width in a radially outward direction in the thrust bearing gap C.