To suppress or prevent rise of bearing torque or abnormal wear at a time of low temperature and at a time of high temperature while maintaining play in a circumference direction small under a condition of use in a ball bearing in a synthetic resin snap cage 4 provided with a dividing part 7 projecting in one side of axial direction at several portions on a circumference of an annular base body 5 and including a pocket 6 for storing ball having a roughly C shape in a view of radial direction between the dividing parts 7.

A shape along a radial direction of an inner wall surface (inner surface of each dividing part 7 and pull-off prevention part 8) constructing each pocket 6 is formed in a spherical surface and a flat part 9 along the radial direction is annularly provided in an inner circumference direction of the pocket 6 in a middle domain in the radial direction of the inner wall surface. Consequently, allowable displacement X1, X2 in the radial direction of a ball 3 in each pocket 6 is set larger than allowable displacement Y1, Y2 in a circumference direction.