A synthetic resin retainer includes two annular members having opposed surfaces opposed to each other and each formed with a plurality of pockets which are circumferentially spaced apart from each other, and in which the balls are received. Each annular member includes axially concave, arc-shaped pocket wall portions defining the inner surfaces of the pockets, and flat plate-shaped coupling plate portions coupling together the adjacent pairs of the pocket wall portions. The coupling plate portions have axial thicknesses of 30% or less of the diameters of the balls, and the pocket wall portions have, at deepest points of the pockets, thicknesses of 10% or less of the diameters of the balls.