A assembly method of assembling a tapered roller bearing for supporting a rotating shaft, the bearing including an inner ring, an outer ring, tapered rollers, and a holder having pockets, the method comprising: positioning the inner ring with a large end surface facing downward; arranging the tapered rollers on a raceway surface; integrating the inner ring, tapered rollers, and holder to form an inner ring assembly; inverting the inner ring assembly such that a small-diameter side faces downward; and inserting the inverted inner ring assembly into the outer ring positioned upward, wherein the tapered rollers are prevented from disengaging from the inner ring.