A synthetic resin-made thrust sliding bearing 1 includes a synthetic resin-made upper casing 100, a synthetic resin-made lower casing 200, and a synthetic resin-made thrust bearing piece 300 interposed between the upper and lower casings 100 and 200. If it is assumed that the radial length of the thrust bearing piece 300 up to its outer peripheral surface 303 is (r), and the thickness of an upper annular plate portion 102 at its inner peripheral surface is (t), the upper casing 100 has on its upper surface 108 an annular flat surface 109 having a circular outer peripheral edge with R in a range of R=r±t.