A retainer for a ball bearing formed in a generally annular shape and having a plurality of pockets arranged in a circumferential direction, respectively, to rollably hold a plurality of balls along a pitch circle, respectively, the pockets each having an inner peripheral surface comprising a radially inner section which is located inward of the pitch circle of the balls in the radial direction of the retainer, and has an inner diameter larger than the diameter of the balls, such that the gap between the radially inner section of the inner peripheral surface of the pockets and the rolling surface of the balls gradually increases toward the opening of the pockets on the inner side in the radial direction of the retainer, and that the maximum inscribing circle of the opening of the pockets on the outer side in the radial direction of the retainer has a diameter smaller than the diameter of the balls.