To acquire a ball bearing having a wide allowance for misalignment, having an elongated life by avoiding premature failure, and capable of being used in a device such as a business machine which inevitably has errors in installation of a bear for the reason of structure in which a mounting error of the ball bearing cannot be decreased because of a structures of the device such as a business machine or the like.

A plurality of balls 4 is arrayed between an inner race 2 and an outer race 3 which are each formed from a bearing steel. The inner race is fitted into a single-diameter shaft and an inner-diameter surface of the inner race is convexly spherical along its axial direction and besides is singly-circular-arc-shaped in its axial cross-section and furthermore a radius of the circular arc is 0.5 to 50 times the ball diameter. A small-diameter roller bearing is thus structured. Grease 7 is sealed between the inner and outer rings and an inner diameter of the inner ring is 4 mm to 50 mm.