A hydraulic dynamic bearing has a rotor comprised of a flanged shaft having a thrust ring portion and a cylinder portion, a hub attached to the flanged shaft, and a rotor magnet attached to an inner peripheral face of a skirt portion of the hub. A stator rotatably supports the rotor and comprises a bearing sleeve to which are attached stator coils, and a stator base plate that supports the sleeve. A gap exists between confronting surfaces of the flanged shaft and the bearing sleeve, and lubricating oil is sealed in the gap. Thrust dynamic pressure generating grooves are formed in one or both faces of the thrust ring, and the grooves are in the form of a herringbone pattern having radial inner groove sections and outer groove sections. The depth of the inner groove sections is shallower than that of the outer groove sections, and the depths of both groove sections are constant. The optimum inner-to-outer groove depth ratio is 0.6 to 0.7.