A strain wave gearing (1, 10, 20), the strain wave gearing (1, 10, 20) comprising: a rigid internally toothed gear (3, 13, 23); a flexible externally toothed gear (6, 16, 26) meshing with the internally toothed gear (3, 13, 23); a wave generator (7, 17, 27) making the externally toothed gear (6, 16, 26) to flex into a non-circular shape and to partially mesh with the internally toothed gear (3, 13, 23); a cylindrical housing (2, 12, 22) to which a first gear is mounted, the first gear being one of the internally toothed gear (3, 13, 23) and the externally toothed gear (6, 16, 26); an output member (5, 25) to which a second gear is mounted, the second gear being the other of the internally toothed gear (3, 13, 23) and the externally toothed gear (6, 16, 26); and a sliding bearing (8, 18, 28) supporting a first part and a second part in a radial direction and in a thrust direction allowing the first and second parts to relatively rotate, the first part including the housing (2, 12, 22) and the first gear, the second part including the second gear and the output member (5, 25) characterized in that the housing (2, 12, 22) and the first gear are a stationary-side part, and the second gear and the output member (5, 25) are a driving-side part; one of the stationary-side part and the driving-side part has an annular convex part (12c, 22c) having a rectangular cross-section and protruding in a radial direction; the other of the stationary-side part and the driving-side part has an annular concave part having a rectangular cross-section, the annular convex part (12c, 22c) being accommodated in the annular concave part from outside or inside in the radial direction; the annular convex part (12c, 22c) and the annular concave part face with each other across a predetermined radial gap in the radial direction, and the annular convex part (12c, 22c) and the annular concave part face with each other across predetermined thrust gaps in a direction of a device axial line (1a, 10a, 20a) on both sides of the annular convex part (12c, 22c) in the direction of the axial line (1a, 10a, 20a); and the sliding bearing (8, 18, 28) has: an outer-ring portion defined by one of the annular convex part (12c, 22c) and the annular concave part; an inner-ring portion defined by the other of the annular convex part (12c, 22c) and the annular concave part; a cylindrical bush (8a, 18a, 28a) accommodated in the radial gap; and annular bushes (8b, 18b, 28b) accommodated in the thrust gaps, respectively. a rigid internally toothed gear (3, 13, 23); a flexible externally toothed gear (6, 16, 26) meshing with the internally toothed gear (3, 13, 23); a wave generator (7, 17, 27) making the externally toothed gear (6, 16, 26) to flex into a non-circular shape and to partially mesh with the internally toothed gear (3, 13, 23); a cylindrical housing (2, 12, 22) to which a first gear is mounted, the first gear being one of the internally toothed gear (3, 13, 23) and the externally toothed gear (6, 16, 26); an output member (5, 25) to which a second gear is mounted, the second gear being the other of the internally toothed gear (3, 13, 23) and the externally toothed gear (6, 16, 26); and a sliding bearing (8, 18, 28) supporting a first part and a second part in a radial direction and in a thrust direction allowing the first and second parts to relatively rotate, the first part including the housing (2, 12, 22) and the first gear, the second part including the second gear and the output member (5, 25) the housing (2, 12, 22) and the first gear are a stationary-side part, and the second gear and the output member (5, 25) are a driving-side part; one of the stationary-side part and the driving-side part has an annular convex part (12c, 22c) having a rectangular cross-section and protruding in a radial direction; the other of the stationary-side part and the driving-side part has an annular concave part having a rectangular cross-section, the annular convex part (12c, 22c) being accommodated in the annular concave part from outside or inside in the radial direction; the annular convex part (12c, 22c) and the annular concave part face with each other across a predetermined radial gap in the radial direction, and the annular convex part (12c, 22c) and the annular concave part face with each other across predetermined thrust gaps in a direction of a device axial line (1a, 10a, 20a) on both sides of the annular convex part (12c, 22c) in the direction of the axial line (1a, 10a, 20a); and the sliding bearing (8, 18, 28) has: an outer-ring portion defined by one of the annular convex part (12c, 22c) and the annular concave part; an inner-ring portion defined by the other of the annular convex part (12c, 22c) and the annular concave part; a cylindrical bush (8a, 18a, 28a) accommodated in the radial gap; and annular bushes (8b, 18b, 28b) accommodated in the thrust gaps, respectively.