A roller bearing comprising an inner ring; an outer ring; a plurality of rollers rollably disposed in an annular space between the inner ring and the outer ring; and a split cage comprising a plurality of cage segments, each of the cage segments having a pair of rim portions (21, 22) that face each other at a prescribed distance in an axial direction of the split cage, and a pair of cage bar portions (23, 24) disposed between the rim portions (21, 22) to connect the rim portions (21, 22) to each other, the cage bar portions (23, 24) and the rim portions (21, 22) defining a single pocket that accommodates a single roller, wherein: turning of each of the cage segments is guided by the roller, the cage segments being arranged in a circular pattern along a circumferential direction of the split cage, in an annular space between the inner ring and the outer ring; a circumferential clearance is provided between adjacent cage segments and radial clearances (S1, S2) are provided between each of the cage segments and the inner ring or the outer ring; each of the cage segments has projections (26, 27, 28, 29) formed so as to project toward at least one of a radially outside and a radially inside, the projections (26, 27, 28, 29) being formed at both the rim portions (21, 22) and the cage bar portions (23, 24), or at the rim portions (21, 22) or the cage bar portions (23, 24) a projecting length (L1,L2,L3,L4) of each of the projections (26, 27, 28, 29) is set to such a length that the projections (26, 27, 28, 29) are brought into contact with a contact face (7,8) on the outer peripheral side portion of the inner ring or on the inner peripheral side portion of the outer ring, toward which the projections (26, 27, 28, 29) project, when the cage segment starts rotating; a pocket-side side face of each of the cage bar portions (23, 24) is a pocket-side face (23a, 24a) that is allowed to be in sliding contact with an outer peripheral face of the roller; the pocket-side face (23a, 24a) has tilted faces, one of the tilted faces (23a2, 24a2) being positioned radially outward of the other one of the tilted faces (23a1, 24a1), the tilted faces (23a2, 24a2) being allowed to be in contact with the outer peripheral face of the roller; characterised in that in a state where an axis of the roller and a rotation axis of the cage segment coincide with each other, a first radial clearance (E1, E2, E3, E4) formed between each of the projections (26, 27, 28, 29) and a contact face (7, 8) on the outer peripheral side portion of the inner ring or on the inner peripheral side portion of the outer ring with which the projection (26, 27, 28, 29) is brought into contact and toward which the projection (26, 27, 28, 29) projects, is larger than a second radial clearance (S6, S7) formed between each of the tilted faces (23a2, 24a2) and the outer peripheral face of the roller. turning of each of the cage segments is guided by the roller, the cage segments being arranged in a circular pattern along a circumferential direction of the split cage, in an annular space between the inner ring and the outer ring; a circumferential clearance is provided between adjacent cage segments and radial clearances (S1, S2) are provided between each of the cage segments and the inner ring or the outer ring; each of the cage segments has projections (26, 27, 28, 29) formed so as to project toward at least one of a radially outside and a radially inside, the projections (26, 27, 28, 29) being formed at both the rim portions (21, 22) and the cage bar portions (23, 24), or at the rim portions (21, 22) or the cage bar portions (23, 24) a projecting length (L1,L2,L3,L4) of each of the projections (26, 27, 28, 29) is set to such a length that the projections (26, 27, 28, 29) are brought into contact with a contact face (7,8) on the outer peripheral side portion of the inner ring or on the inner peripheral side portion of the outer ring, toward which the projections (26, 27, 28, 29) project, when the cage segment starts rotating; a pocket-side side face of each of the cage bar portions (23, 24) is a pocket-side face (23a, 24a) that is allowed to be in sliding contact with an outer peripheral face of the roller; the pocket-side face (23a, 24a) has tilted faces, one of the tilted faces (23a2, 24a2) being positioned radially outward of the other one of the tilted faces (23a1, 24a1), the tilted faces (23a2, 24a2) being allowed to be in contact with the outer peripheral face of the roller; characterised in that in a state where an axis of the roller and a rotation axis of the cage segment coincide with each other, a first radial clearance (E1, E2, E3, E4) formed between each of the projections (26, 27, 28, 29) and a contact face (7, 8) on the outer peripheral side portion of the inner ring or on the inner peripheral side portion of the outer ring with which the projection (26, 27, 28, 29) is brought into contact and toward which the projection (26, 27, 28, 29) projects, is larger than a second radial clearance (S6, S7) formed between each of the tilted faces (23a2, 24a2) and the outer peripheral face of the roller.