To provide an internal combustion engine wherein occurrence of a phenomenon of twisting of a crankshaft can be reduced.

In the crankshaft 10 of the internal combustion engine 100, a linear reciprocating motion of a piston is converted to a rotational motion by means of the crankshaft 10. The crankshaft 10 and a balance shaft 60 provided parallel therewith are synchronized with each other respectively at both ends by a balance-shaft gear 90 and a crankshaft gear 80. That is, the twist phenomenon of the crankshaft 10 is restrained by the rigidity of the balance shaft 60.

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