A roller bearing for use at a bearing temperature within a predetermined temperature range of -40 to 150°C, the roller bearing comprising: an outer ring and an inner ring both made of a steel material; a plurality of rollers disposed between the outer ring and the inner ring; pockets in which the respective rollers are received; and a plurality of cage segments circumferentially continuously arranged to form an annular cage between the outer ring and the inner ring, the cage segments including a firstly mounted cage segment that is mounted first and a lastly mounted cage segment that is mounted last, wherein the plurality of cage segments are arranged such that a gap is defined, at the lower limit value of the predetermined temperature range, between the firstly mounted cage segment and the lastly mounted cage segment, and such that no gap is defined between any adjacent pair of the cage segments except between the firstly mounted cage segment and the lastly mounted cage segment, and wherein a circumferential dimension of the gap is, at the upper limit value of the predetermined temperature range, less than 0.12% of a circumference of a circle passing through centers of the cage segments of the annular cage in radial thickness directions of the cage segments, characterized in that the cage segments are made of a resin containing a thermally expandable filler such that the resin and the thermally expandable filler have, in combination, a coefficient of linear expansion of less than 1.0 × 10 -5 /°C, and the circumferential dimension of the gap is, at the lower limit value of the predetermined temperature range, more than 0 and 0.075 % or less of the circumference of the circle passing through the centers of the cage segments of the annular cage in the radial thickness directions of the cage segments. an outer ring and an inner ring both made of a steel material; a plurality of rollers disposed between the outer ring and the inner ring; pockets in which the respective rollers are received; and a plurality of cage segments circumferentially continuously arranged to form an annular cage between the outer ring and the inner ring, the cage segments including a firstly mounted cage segment that is mounted first and a lastly mounted cage segment that is mounted last, wherein the plurality of cage segments are arranged such that a gap is defined, at the lower limit value of the predetermined temperature range, between the firstly mounted cage segment and the lastly mounted cage segment, and such that no gap is defined between any adjacent pair of the cage segments except between the firstly mounted cage segment and the lastly mounted cage segment, and wherein a circumferential dimension of the gap is, at the upper limit value of the predetermined temperature range, less than 0.12% of a circumference of a circle passing through centers of the cage segments of the annular cage in radial thickness directions of the cage segments, characterized in that the cage segments are made of a resin containing a thermally expandable filler such that the resin and the thermally expandable filler have, in combination, a coefficient of linear expansion of less than 1.0 × 10 -5 /°C, and the circumferential dimension of the gap is, at the lower limit value of the predetermined temperature range, more than 0 and 0.075 % or less of the circumference of the circle passing through the centers of the cage segments of the annular cage in the radial thickness directions of the cage segments.