An elastic member (2) is fixed to a crankshaft (1), and a flywheel (5) is fixed to the elastic member (2) so that the flywheel is supported in an elastic relationship with the crankshaft. The elastic member (2) has a rigidity in its rotating direction large enough to effectively transmit driving power to a transmission through a clutch. On the other hand, the elastic member (2) has a rigidity in an axial direction of the crankshaft small enough (e.g. 600 to 2200 kg/mm) to shift the resonance frequency of bending vibration out of a target frequency band of forced vibration (e.g. 200 to 500 Hz), while ensuring prevention of failure of disengagement of the clutch. The flywheel (5) has a clutch-engageable surface (5g) whose axial run-out is no more than 0.1 mm.
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