The variable compression ratio mechanism 1, 1′ comprises: a connecting rod 20 provided with a crank receiving opening 23; an eccentric member 30, 30′ provided with a piston pin receiving opening 36 and attached to the connecting rod to be able to turn so as to change a length between a center axial line of the piston pin receiving opening and a center axial line of the crank receiving opening; a swing member 40, 50, 40′ attached to the connecting rod to be able to swing and able to engage with the eccentric member; and an actuating member 60, 60′ swinging the swing member from an initial position toward a stop position. The swing member engages with the eccentric member to turn the eccentric member when swinging from the initial position toward the stop position, and returns from the stop position to the initial position when disengaged form the eccentric member.