A positioning protrusion 14 is provided in a center of an outer diameter surface of a halved outer ring 11a in both circumferential and axial directions. The positioning protrusion 14 is configured to engage with a housing, which is placed on an outer diameter side of the halved outer ring, to position the halved outer ring 11a with respect to the housing. A protruding portion 17a and a recessed portion 17b are provided in each abutting portions 16a, 16b where a pair of the halved outer rings 11a abut on each other when fitted on a rotary shaft that is supported by a roller bearing. The protruding portion 17a and the recessed portion 17b are configured to mesh with each other to restrict axial movement of both halved outer rings 11a. The protruding portion 17a and the recessed portion 17b are point-symmetric with respect to a center 20 of the positioning protrusion 14.