The invention discloses an internal combustion engine provided with a crank gear, a balancing gear and an intermediate gear. The crank gear is fixed on a crank that supported on an engine noumenon in a freely rotating way; the balancing gear is fixed on a balancing shaft set on the engine noumenon that can rotate freely; the intermediate gear transmits the rotation of the crank gear to the balancing gear by engaging the crank gear with the balancing gear; a gear box contains the intermediate gear and the balancing gear, and is mounted on lateral part of the engine noumenon; a positioning partis set between the engine noumenon and the gear box, and positions location of rotating center of the intermediate gear.