A synthetic resin-made sliding bearing comprising: a synthetic resin-made upper casing, a reinforced synthetic resin-made lower casing superposed on said upper casing so as to be rotatable about an axis relative to said upper casing, and a synthetic resin-made sliding bearing piece disposed between said upper casing and said lower casing, wherein said upper casing includes: an annular upper casing base portion having an annular lower surface; an inner peripheral-side cylindrical suspended portion suspended from a radial inner peripheral end portion of the annular lower surface of the upper casing base portion; an outer peripheral-side cylindrical suspended portion suspended from a radial outer peripheral end portion of the annular lower surface of the upper casing base portion; an annular upper flat portion extending radially outwardly from a lower end portion of the outer peripheral-side cylindrical suspended portion; an annular engaging suspended portion suspended from a radially outer peripheral end portion of the annular upper flat portion; and an engaging bulged portion having an annular inclined engaging surface and bulging radially inwardly from a radially inner surface of a lower portion of the annular engaging suspended portion, said lower casing includes: an annular lower casing base portion which has an annular upper surface and a cylindrical inner peripheral surface; a cylindrical protruding portion which has a cylindrical inner peripheral surface; an annular protruding portion which protrudes upwardly from the annular upper surface of the lower casing base portion; an annular lower flat portion extending radially outwardly; an annular engaged protruding portion which protrudes upwardly from a radially outer peripheral end portion of the annular lower flat portion; and an engaged bulged portion bulging radially outwardly from the annular engaged protruding portion and having an annular inclined engaged surface, and wherein said sliding bearing piece includes: an annular thrust sliding bearing piece portion having an annular upper surface, and an annular lower surface, characterized in that the cylindrical inner peripheral surface is continuously connected to the cylindrical inner peripheral surface of the lower casing base portion and protrudes downwardly from a lower surface of a radially inner peripheral end portion of the lower casing base portion; the annular lower flat portion extends radially outwardly from a radially outer peripheral lower end portion of the lower casing base portion; the engaged bulged portion bulges radially outwardly from a radially outer surface of an upper portion of the annular engaged protruding portion; and the lower casing further includes at least one engaging hole portion which is formed in an annular upper surface of the annular protruding portion so as to be open at the annular upper surface and to extend downwardly from the annular upper surface; the sliding bearing piece further includes at least one engaging protruding portion which projects downwardly from the annular lower surface and is fittingly inserted in the engaging hole portion of said lower casing; and a cylindrical radial sliding bearing piece portion which, at an one end portion thereof, is integrally formed on one annular end portion of the thrust sliding bearing piece portion in such a manner as to extend downwardly from the one annular end portion of the thrust sliding bearing piece portion, and has a cylindrical inner peripheral surface and a cylindrical outer peripheral surface, said sliding bearing piece being disposed between said upper casing and said lower casing such that the annular upper surface of the thrust sliding bearing piece portion is brought into slidable contact with the annular lower surface of the upper casing base portion, and the annular lower surface of the thrust sliding bearing piece portion is brought into contact with the annular upper surface of the annular protruding portion, and such that the cylindrical inner peripheral surface of the radial sliding bearing piece portion is brought into contact with a cylindrical outer peripheral surface of the lower casing base portion and a cylindrical outer peripheral surface of the annular protruding portion, and the cylindrical outer peripheral surface of the radial sliding bearing piece portion is brought into slidable contact with a cylindrical inner peripheral surface of the outer peripheral-side cylindrical suspended portion, said upper casing being combined with said lower casing by causing the inclined engaging surface to be resiliently fitted to the inclined engaged surface of said lower casing.