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 (R) 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 (R) 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 (R) 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 (R) 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 (R) 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 (R) 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 . <EOS>