An ultrathin cross-roller bearing composed of outer and inner rings, more than one roller rolling through between the outer and inner rings, and a separator interposed between any two adjoining rollers. With the ultrathin cross-roller bearing constructed as stated earlier, the separator lying between the adjoining rollers is in the form of right cylinder better for keeping constantly the roller in proper posture, getting the bearing itself downsized and lightweight as small as possible, and further making sure of positive lubrication. The outer and inner rings are each made into one-piece construction. The inner ring has an inside diameter in a range of from 20 mm to 50 mm. The roller used in the ultrathin cross-roller bearing has a diameter of 2 mm to render a transverse section given by a product of a radial height and a bearing width in transverse section small or ultrathin in thickness.