A synthetic resin-made sliding bearing comprising: a synthetic resin-made upper casing, a synthetic resin-made lower casing superposed on said upper casing so as to be rotatable about an axis in a circumferential direction relative to said upper casing, and a synthetic resin-made sliding bearing piece disposed in a space between said upper casing and said lower casing, wherein said upper casing integrally includes: 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 radial inner peripheral end portion of the upper casing base portion; an outer peripheral-side cylindrical suspended portion suspended from a radial outer peripheral end portion of the upper casing base portion; an intermediate cylindrical suspended portion forming an annular recessed portion in cooperation with an inner surface of the outer peripheral-side cylindrical suspended portion and suspended downwardly from the annular lower surface; and an engaging hook portion which protrudes radially inwardly from an inner peripheral surface of an end portion of the outer peripheral-side cylindrical suspended portion, wherein said lower casing integrally includes: an annular lower casing base portion which has an annular upper surface in the axial direction; a central cylindrical protruding portion protruding axially upwardly from the annular upper surface of the lower casing base portion; a cylindrical portion protruding axially downwardly from an inner peripheral portion of an annular lower surface of the lower casing base portion; an intermediate cylindrical protrusion protruding axially upwardly from an outer peripheral edge of an annular upper surface of the central cylindrical protruding portion; an outer cylindrical protruding portion which protrudes radially outwardly from an outer peripheral surface of the central cylindrical protruding portion so as to form an outer annular recessed portion on the outer peripheral surface of the central cylindrical protruding portion in cooperation with an outer peripheral surface of the intermediate cylindrical protrusion, and which has on its outer peripheral surface an enlarged-diameter tapered surface which is gradually enlarged in diameter toward an axially lower side, a reduced-diameter tapered surface which is continuously connected to the enlarged-diameter tapered surface, forms an engaged hook portion in cooperation with the enlarged-diameter tapered surface, and is gradually reduced in diameter toward the axially lower side, and a curved recessed surface continuously connected to the reduced-diameter tapered surface; and a wide annular flange portion which is formed on the outer peripheral surface of the central cylindrical protruding portion in such a manner as to extend radially outwardly such that its upper surface is continuously connected to the curved recessed surface and its lower surface is flush with the annular lower surface of the lower casing base portion, wherein said sliding bearing piece is disposed in the space including an annular space between the annular lower surface of the upper casing base portion and the annular upper surface of the central cylindrical protruding portion and a cylindrical space between an outer peripheral surface of the inner peripheral-side cylindrical suspended portion and a cylindrical inner peripheral surface of the central cylindrical protruding 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 cylindrical suspended portion, respectively, 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 of the central cylindrical protruding portion and a radial cylindrical inner peripheral surface of the central cylindrical protruding portion, respectively, and wherein said upper casing is superposed on said lower casing so as to be rotatable about the axis in the circumferential direction, such that an outer sealing portion for performing labyrinth action is formed by disposing the intermediate cylindrical suspended portion in the outer annular recessed portion of said lower casing and by disposing an axial upper end portion of the outer cylindrical protruding portion of said lower casing in the annular recessed portion, and such that the engaging hook portion is resiliently fitted to the engaged hook portion, and a clearance formed by the curved recessed surface and the engaging hook portion in a resiliently fitting section including the engaging hook portion and the engaged hook portion is covered by the wide annular flange portion of said lower casing downward in the axial direction, characterized in that, said upper surface of the wide annular flange portion has an annular flat surface portion and a tapered surface portion which is continuously connected to the annular flat surface portion and having a downward gradient extending toward an outer peripheral surface of the annular flange portion, and wherein a plurality of through holes each having one end open at the tapered surface portion and another end open at the lower surface are formed in an outer peripheral edge side of the wide annular flange portion along the circumferential direction.