PURPOSE: To form compression ratio of an engine changeable in an analog state, by rotatably supporting a rotary connection part and the like of a connecting rod through eccentric bearings rotatable themselves and rotatively displacing said eccentric bearings in accordance with an operational condition of the engine.

CONSTITUTION: A connecting rod 2 is connected to a piston 1 and rotatably connected to a crank pin part 3a of a crankshaft 3, and journal parts 3b of the crankshaft 3 are supported to bearing parts 5 in a cylinder block through eccentric bearings 4. The eccentric bearings 4 are formed rotatable for the bearing part 5, and a tooth 4a is notched to the peripheral surface of the bearing. While a worm gear 6 is mounted through ball bearings 7 in the bearing part 5, then said gear 6 is both meshed to the above described tooth 4a and connected to a pulse motor 8. Then the pulse motor 8 is driven and controlled through a pulse generator 9, controlled by a CPU10 in accordance with outputs of each sensor 11W13 of intake negative pressure, water temperature and speed.