A variable compression ratio device including a piston, a crankshaft, and a connecting rod may include an eccentric bearing that contacts an exterior circumference of the piston pin, wherein an exterior circumference of the eccentric bearing contacts an interior circumference of a ring formed at the small end of the connecting rod, a ratchet rod slidably coupled inside the piston pin and movable along a longitudinal direction, a variable slider that selectively contacts one of both ends of the ratchet rod, and that is disposed at both sides of a cylinder for pushing the one of both ends of the ratchet rod in an opposite direction, and a guide plate for reciprocally moving both variable sliders along a vertical direction of the ratchet rod.