PURPOSE: To easily change compression ratio by so arranging a crankshaft that its axial line can rotate about crankcasing and other axial line so as to change the degree of eccentricity.

CONSTITUTION: A crankshaft 29 is supported by an upper and a lower support elements 30, 41 so that its axial line B can rotates about the axial line A. By, for example, anti-clockwise rotating these support elements 30, 41, the crankshaft 29 is raised upward, and the top of a piston approaches the bottom of the cylinder head, so the compression ratio is increased. Conversely, while the support elements 30, 41 is rotated clockwise, the compression ratio is reduced. Therefore, the rotation of the crankcasing 20 rotating about the axial line A is conducted by a gear 55 that is driven by a direct current motor 56 through a one-way reducer and its action is controlled by output of position detectors 58 to 60.