A synthetic resin-made thrust sliding bearing comprising: a synthetic resin-made upper casing integrally including an annular upper casing base portion having an annular lower surface in an axial direction, an inner peripheral-side cylindrical suspended portion suspended from a radially inner peripheral end portion of the annular lower surface of the upper casing base portion, and an outer peripheral-side cylindrical suspended portion suspended from a radially outer peripheral end portion of the annular lower surface of the upper casing base portion; a synthetic resin-made lower casing integrally including an annular lower casing base portion having an annular upper surface in the axial direction and superposed on said upper casing so as to be rotatable about an axis relative to said upper casing and an annular protrusion protruding from the annular upper surface of the lower casing base portion toward the annular lower surface of the upper casing base portion; a synthetic resin-made sliding bearing piece disposed in an annular space between the annular lower surface of the upper casing base portion and an annular upper surface of the annular protrusion of the lower casing base portion and in an annular space between an outer peripheral surface of the inner peripheral-side cylindrical suspended portion and a cylindrical inner peripheral surface of the annular protrusion, so as to be brought at an axial annular upper surface and a radial cylindrical inner peripheral surface thereof into slidable contact with the annular lower surface of the upper casing base portion and a radial outer peripheral surface of the inner peripheral-side cylindrical suspended portion, while being brought at an axial annular lower surface and a radial cylindrical outer peripheral surface thereof into contact with an axial annular upper surface and a radial cylindrical inner peripheral surface of the annular protrusion, characterized in that, said synthetic resin-made thrust sliding bearing further including a synthetic resin-made seal member having on a radially inner peripheral side thereof a flexible inner peripheral-side annular seal portion which is brought into contact with a radially outer peripheral surface of the inner peripheral-side cylindrical suspended portion of said upper casing so as to seal a gap between the inner peripheral-side cylindrical suspended portion of said upper casing and the annular protrusion of the lower casing base portion, having on a radially outer peripheral side thereof a flexible outer peripheral-side annular seal portion which is brought into contact with a radially inner peripheral surface of the outer peripheral-side cylindrical suspended portion of said upper casing so as to seal a gap between the outer peripheral-side cylindrical suspended portion of said upper casing and the annular protrusion of the lower casing base portion, and the synthetic resin-made seal member further having connecting portions for connecting to each other the inner peripheral-side annular seal portion and the outer peripheral-side annular seal portion, wherein the inner peripheral-side annular seal portion and the outer peripheral-side annular seal portion are integrally molded with the connecting portions having respective columnar portions arranged in a plurality of hole portions in the annular upper surface of the annular protrusion of the lower casing base portion, and wherein the inner peripheral-side annular seal portion includes an annular inner peripheral seal base portion which is joined to a cylindrical inner surface of the annular protrusion of the lower casing base portion in such a manner as to cover a plurality of protrusions which are integrally formed on the cylindrical inner surface of the annular protrusion along a circumferential direction, and the inner peripheral-side annular seal portion further includes a flexible inner peripheral seal portion which is connected to a radial inner peripheral end portion of the inner peripheral seal base portion and which is elastically brought into flexural contact with an outer peripheral surface of the inner peripheral-side cylindrical suspended portion of said upper casing, and wherein the inner peripheral seal portion has a smaller thickness than the thickness of the inner peripheral seal base portion and extends diagonally downwardly from an outer peripheral end portion connected to the inner peripheral end portion of the inner peripheral seal base portion, and/or wherein the outer peripheral-side annular seal portion includes an annular outer peripheral seal base portion which is joined to an outer peripheral surface continuing to an annular flat portion of the annular protrusion of the lower casing base portion in such a manner as to cover the outer peripheral surface and a plurality of protrusions formed integrally on the outer peripheral surface along the circumferential direction, and the outer peripheral-side annular seal portion further includes a flexible outer peripheral seal portion which is connected to the outer peripheral seal base portion and is elastically brought into flexural contact with an inner peripheral surface of a cross-sectionally trapezoidal cylindrical portion of the outer peripheral-side cylindrical suspended portion, and wherein the outer peripheral seal portion has a smaller thickness than the thickness of the outer peripheral seal base portion and extends diagonally downwardly from an inner peripheral end portion connected to the outer peripheral end portion of the outer peripheral seal base portion.