A synthetic resin-made thrust 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 an annular clearance formed by the central cylindrical protruding portion and the outer peripheral-side cylindrical suspended portion is covered by the wide annular flange portion of said lower casing downward in an axial direction, and wherein said sliding bearing piece includes: an annular thrust sliding bearing piece portion; and a cylindrical radial sliding bearing piece portion, which, at its one annular end portion, is integrally formed on one annular end portion of the thrust sliding bearing piece portion in such a manner as to extend from that one end portion downwardly in the axial direction, characterized in that, the thrust sliding bearing piece portion has plurality of inner recessed portions and plurality of outer recessed portions, which are formed in its annular upper surface along the circumferential direction and in at least two rows including an inner row and an outer row in the radial direction, the inner recessed portions and the outer recessed portions are arranged with phase differences with respect to each other in the circumferential direction, and the radial sliding bearing piece portion has a plurality of axial grooves, which are open at their both ends and are formed on the cylindrical inner peripheral surface by being spaced apart at equal intervals in the circumferential direction, and wherein each of the plurality of inner recessed portions is defined by an inner circular arc-shaped wall surface; an outer circular arc-shaped wall surface; a pair of semicircular wall surfaces respectively continuously connected to the inner circular arc-shaped wall surface and the outer circular arc-shaped wall surface; and a bottom wall surface continuously connected to respective ones of the inner circular arc-shaped wall surface, the outer circular arc-shaped wall surface, and the pair of semicircular wall surfaces.