Provided are a retainer capable of imparting a torque reduction effect even at low cost and a ball bearing (deep-groove ball bearing) having the retainer assembled thereto. The retainer includes two annular retaining plates (27A, 28B) combined with each other, the two annular retaining plates each having semispherical bulging portions (26) arranged at predetermined intervals along a circumferential direction of the retainer, the semispherical bulging portions (26, 26) of the two annular retaining plates, which face each other, forming a pocket (30) having a ring-like shape, for retaining a ball (16). The pocket (30) includes a ball non-contact portion (32) formed in a ball facing surface of the pocket (30) at a center portion in a pocket axial direction, the ball non-contact portion (32) being defined by a recess (34) extending in a pocket circumferential direction. The following relationship is set: A/(B+C)=0.70 to 0.90, where "A" represents a pocket circumferential length of the ball non-contact portion (32), "B" represents a diameter of the ball, and "C" represents a gap formed between the ball (16) and the ball facing surface of the pocket (30).