PURPOSE: To enhance the durability of an output side gear and to reduce mechanical sound, by associatingly coupling two parallel crankshafts rotated in the one and the same direction with the use of a chain transmission mechanism, and by providing a power take-off gear onto either one of the crankshaft.

CONSTITUTION: Two parallel crankshafts 1, 2 are coupled together at the same side ends thereof with a chain transmission mechanism 5. Further, the above- mentioned transmission mechanism 5 is composed of two chain sprockets 6, 7 and a transmission chain 8. An output power take-off drive gear 10 is fixed onto either one of the crankshafts 2, and is meshed with an output side clutch gear 3. Further, one 6 of the sprockets is provided thereto with a damper mechanism 11 through which the sprocket 6 and the crankshaft 1 are coupled together. This damper mechanism consisting of a rotor 12, a coil spring 13, etc., and the chain sprocket 6 is fitted onto the outer periphery of the rotor 12 through the intermediary of a bearing bushing 16.