PURPOSE: To smoothly switch the compression ratio with a suppressed impact force by pressing the small end section of a connecting rod downward, and restricting the rotation of an eccentric piston when the low-compression ratio is switched to the high-compression ratio.

CONSTITUTION: A pressing mechanism 24 for pressing the small end section 7 of a connecting rod 6 downward and restricting the rotation of an eccentric piston pin 5 is provided between the small end section 7 connected to the eccentric piston pin 5 and the piston head 3a of a piston 3. When the high oil pressure is fed to the hydraulic chamber 27 of the pressing mechanism 24 via the control of a solenoid valve 23 at the position near the intake upper dead point of a crank angle, the small end section 7 is pressed downward by an auxiliary piston 26 pressed downward, and the compression ratio is gradually transferred to the high-compression ratio state. The eccentric piston pin 5 is kept at the rotation restricted state at this time, thus the first lock pin 10 can be smoothly coupled with a pin hole 11, and the impact force applied to the first lock pin 10 is suppressed nearly to zero.