A ball bearing for a small size fan for supporting an axis of rotation of an impeller is formed of four or five balls (21), a crown-shaped cage (25), an inner ring (22), and an outer ring (23). By using an optimized number of the balls (21) under the working condition of a ball bearing for a small size fan, structually useless portions are eliminated. Further, by adopting the crown-shaped cage (25), reduction of the cost and the friction torque is attained, and a ball bearing having necessary and sufficinet durability is provided. By incorporating the ball bearing in a small size fan, reduction of the cost, simplification of the structure, and reduction of the electric power consumption of the small size fan are facilitated, thereby improving the commerciality as a small size fan.