To obtain a compression ratio corresponding to a load of an engine while making an expansion ratio larger than the compression ratio, and to continuously change the compression ratio.

This engine comprises an eccentric shaft 15 in parallel to a crank shaft 2, and a drive device 8 for rotating the eccentric shaft 15 at a half of a rotation speed of the crank shaft 2. The engine has a link 5 interposed between a crank pin 3 and a large end 4a of a connection rod 4, and a lever 7 connecting the link 5 with the eccentric shaft 15. The drive device 8 is provided with: a planetary gear decelerator 21, and a phase changing device 23 for changing a rotation phase of the eccentric shaft 15 by changing an actuating condition of the planetary gear decelerator 21.