Abstract: An object is to exert the intended fretting-preventive effect without being influenced by the size of variation in rotation (variation in speed) or the size of vibration. An arrangement comprising a main shaft gear 1 having a raceway surface 1c on its inner peripheral surface, a bearing inner ring 2 having a raceway surface 2a on the outer periphery, tapered rollers 3 disposed between the raceway surface 1c of the main shaft gear 1 and the raceway surface 2a of the bearing inner ring 2 and held at predetermined circumferential intervals by a cage 4, an imbalance means which produces a circumferential weight imbalance in the assembly of the tapered rollers 3 and cage 4, wherein the roughness of the cone back face 2b of the bearing inner ring 2 is 0.01-0.04 &mgr;mRa, and the roughness of the large end faces 3b of the tapered rollers 3 is 0.01-0.