upper end of each vertical line represents the maximum of the noise produced the lower end the minimum and the bullet in the middle an average as is apparent from fig5 the ball bearing of example has excellent acoustic characteristics the above test results lent confirmation to the fact that the ball bearing of the present invention has a small turning torque with little fluctuation and exhibits excellent acoustic characteristics as described above the injection molding material used in the present invention is a mixture of a synthetic resin especially a polyolefin resin having a high oil absorption and a lubricating oil and therefore retains by itself a large quantity of a lubricating oil on being molded and cooled the molding material solidifies while including the lubricating oil in the synthetic resin matrix to provide a molded article maintaining the initial oil content thus the resulting molded article also retains a large quantity of the lubricating oil unlike the conventional cage which is obtained by immersing a molded article in a lubricating oil and therefore retains much oil only in the vicinity of its surface oozing or release of the lubricating oil from the cage of the present invention is sustained whereby a stable lubricating action is exerted for an extended period of time further since the cage can be produced by injection molding like general synthetic resins such as polyamide resins the bearing of the present invention is substantially as inexpensive as general ball bearings since no post treatment for furnishing a cage with a lubricating oil is required not only is reduced the time of production but product quality is uniform with no.