PURPOSE: To vary a compression ratio according to a driving state as well as to aim at improvement in output, abatement in fuel consumption and emission control over exhaust gas by making a crankshaft shiftable to an engine body, in the case of a reciprocating type internal combustion engine for an automobile or the like.

CONSTITUTION: Variations in engine load are detected by suction pressure, a stepping motor or the like is driven in response to this detection, a worm wheel 7 is rotated by a worm gear 6, and with eccentric movement of a crankshaft being eccentrically held by this worm wheel, moving the motion area of a piston 1 via a connecting rod, thus a compression ratio is varied. And, engine output, namely, rotation of the crankshaft 3 is taken off via an inner hear being fixed to this crankshaft and an outer gear 5 being engaged with this inner gear. Thus, the compression ratio is variable according to a driving state, so that improvement in highly efficient output, abatement in fuel consumption and emission control over exhaust gas are made promotable all the time.