A tapered roller bearing is capable of suppressing the occurrence of fretting without shortening its fatigue life. The tapered roller bearing includes an outer race, an inner race, a plurality of taper rollers interposed between the outer race and the inner race, and a retainer for retaining the tapered rollers. The tapered roller bearing is used in an idler portion of an automobile transmission, which is interchanged between an idle state in which the outer race idles with respect to the inner race and a shift state in which the outer race and the inner race are rotated synchronously with each other. A ratio of α/β≧4.9 is established on a premise that an outer-race raceway surface angle is 2α and a roller angle is β.