A synthetic resin-made sliding bearing 1 includes a synthetic resin-made upper casing 2 which is fixed to a vehicle body side through a mounting member; a reinforced synthetic resin-made lower casing 3 which is superposed on the upper casing 2 so as to be rotatable about an axis O in a circumferential direction R relative to the upper casing 2; and a synthetic resin-made sliding bearing piece 5 disposed in a space 4 between the upper casing 2 and the lower casing 3.