PURPOSE:To achieve reduction of drive resistance and restriction of vibration by making the center of the total mass of a pair of unbalance portions come near the center of a second journal and reducing the amount of deflection between a first journal and the second journal. CONSTITUTION:A pair of unbalance portions 8e, 8f are provided on opposite sides of a second journal 8c, the axial length l1 of the unbalance portion 8e adjacent to a first journal 8b being longer than the axial length l2 of the other unbalance portion 8f. An unbalance mass 8g is provided at a portion adjacent to the first journal on the antiunbalanced side. This unbalance mass 8g allows the center of the total mass of the pair of unbalance portions 8e, 8f to come near the center of the second journal 8c corresponding to the axial center of the engine, permitting effective restriction of vibration of a balancer shaft.
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