A gear drive mechanism is provided with a gear drive pin provided at the second gear, a first movable member and second movable member configured to be able to move to the gear drive pin side, a first guide groove formed at the first movable member and engaged with the gear drive pin when making the eccentric member rotate toward a low compression ratio position, and a second guide groove formed at the second movable member and engaging with the gear drive pin when making the eccentric member rotate toward the high compression ratio position and wherein the mechanism makes the gear drive pin moving relative to a counterweight move along the first guide groove and the second guide groove so as to make the second gear rotate.