A 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 suspended cylindrical portion suspended from a radial inner peripheral end portion of the annular lower surface of the upper casing base portion, and an outer peripheral-side suspended cylindrical portion suspended from a radial 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 thrust sliding bearing piece disposed in an annular space between the annular lower surface of the upper casing base portion and the annular upper surface of the lower casing base portion, 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 suspended cylindrical 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; and a synthetic resin-made seal member having on a radial inner peripheral side thereof a flexible inner peripheral-side annular seal portion which is brought into contact with a radial outer peripheral surface of the inner peripheral-side suspended cylindrical portion of the upper casing so as to seal a gap between the inner peripheral-side suspended cylindrical portion of said upper casing and the lower casing base portion of said lower casing, having on a radial outer peripheral side thereof a flexible outer peripheral-side annular seal portion which is brought into contact with a radial inner peripheral surface of the outer peripheral-side suspended cylindrical portion of said upper casing so as to seal a gap between the outer peripheral-side suspended cylindrical portion of said upper casing and the lower casing base portion of said lower casing, and having a plurality of connecting portions arranged by being spaced apart from each other in a direction about the axis, so as to connect to each other the inner peripheral-side annular seal portion and the outer peripheral-side annular seal portion, wherein each of the plurality of connecting portions is passed between said thrust sliding bearing piece and the annular protrusion of said lower casing and is integrally connected at one end portion thereof to the inner peripheral-side annular seal portion and at another end portion thereof to the outer peripheral-side annular seal portion, wherein each of the connecting portions of said seal member has a radial connecting portion which is integrally connected at one radial end portion thereof to the outer peripheral-side annular seal portion and an axial connecting portion which is integrally connected at one axial end portion thereof to another radial end portion of the radial connecting portion and at another axial end portion thereof to the inner peripheral-side annular seal portion, wherein the annular protrusion of said lower casing has upper surface grooves which are arranged on an axial upper surface thereof by being spaced apart from each other in a direction about the axis and receive respective ones of the radial connecting portions of the connecting portions and inner peripheral surface grooves which are arranged on a radial inner peripheral surface thereof by being spaced apart from each other in the direction about the axis and connected to respective ones of the upper surface grooves and receive respective ones of the axial connecting portions of the connecting portions, wherein the upper surface of the annular protrusion of the lower casing excluding the upper surface grooves is positioned flush with or higher than an axial upper surface of the radial connecting portion of the connecting portion, and the inner peripheral surface of the annular protrusion of the lower casing excluding the inner peripheral surface grooves is positioned flush with or radially inwardly of a radial inner side surface of the axial connecting portion of the connecting portion, and wherein said thrust sliding bearing piece has a radial bearing piece portion which is in contact with the upper surface of the annular protrusion of the lower casing and an axial bearing piece portion which is in contact with the inner peripheral surface of the annular protrusion of the lower casing.