A sliding bearing includes: an upper casing; a synthetic resin-made lower casing which is superposed on the upper casing so as to be rotatable about an axis in a circumferential direction relative to the upper casing; a synthetic resin-made sliding bearing piece disposed in an annular space between the upper casing and the lower casing; and a synthetic resin-made seal member for sealing respective other annular end portions, communicating with the outside, of a gap on an inner peripheral side and a gap on an outer peripheral side in a radial direction between the upper casing and the lower casing, the gap and the gap respectively communicating with the annular space at their one annular end portions.