PURPOSE: To enable stepless change of the length of stroke and phase change by fixing a follower planetary gear to an eccentric ring pivoted on the outer ring of connection rod, supporting it with a crank pin, and providing a drive planetary gear.

CONSTITUTION: A follower planetary gear 21 and an eccentric ring 22 fixed thereto are mounted on the crank pin 16 of a U-shaped crank arm 15 driven by a crank shaft 14 and a connection rod 17 is pivoted on an eccentric ring outer circumference 24 by means of an outer ring 23, while a drive planetary gear 25 is provided on the inside of the arm 15. A control means 26 is provided by which a shaft 27 fixing a drive planetary gear 25 is driven through a worm wheel 28, worm 29 and the relative positions of an eccentric ring 22 and the outer ring 23 are changed. With this arrangement, stepless increase/decrease in stroke is made possible and this piston crank system improves combustion efficiency if applied to a four-cycle engine.