To provide a variable compression ratio mechanism of an internal combustion engine with changeable compression ratio for each cylinder.

The variable compression ratio mechanism includes: an inner piston 14 connected to a crankshaft via a connecting rod 12; an outer piston 15 slidably fitted on the inner piston 14; a pressure working chamber 16 formed between a top back face 15a of the outer piston 15 and a top front surface 14a of the inner piston 14; a working fluid supply and discharge passage 17 provided in the connecting rod 12 for supplying and discharging working fluid to the pressure working chamber 16; an opening and closing valve 26 disposed in the working fluid supplying and discharging passage 17 for opening and closing the working fluid supplying and discharging passage 17; and a control part 22 for controlling opening and closing of the opening and closing valve 26.