A plain bearing (1) provided with the following: an upper case (2); a synthetic-resin lower case (3) that is placed up against the upper case (2) so as to be able to rotate with respect thereto in a circumferential direction (R) about a central axis (O); a synthetic-resin plain-bearing piece (5) disposed in a ring-shaped space (4) between the upper case (2) and the lower case (3); and a synthetic-resin seal member (8). There are two ring-shaped gaps between the upper case (2) and the lower case (3): one (6) on the inside in a radial direction (X) and one (7) on the outside in said direction. One side of each of said gaps connects to the abovementioned ring-shaped space (4), and the other side of each gap connects to the outside but is blocked by the synthetic-resin seal member.