A variable compression ratio device is provided. The device includes a connecting rod connected to a piston in a cylinder and an eccentric cam interposed between a piston pin and the connecting rod. The eccentric cam moves the piston vertically to vary a compression ratio according to an eccentric cam rotation position. A hydraulic controller adjusts hydraulic fluid supplied to first and second chambers formed between the eccentric cam and the connecting rod. The first chamber rotates the eccentric cam in one direction by supplied hydraulic fluid. The second chamber rotates the eccentric cam in another direction by the supplied hydraulic fluid. A position sensor senses a piston position. A compression ratio controller calculates a target compression ratio according to an operating condition of an engine, calculates a current compression ratio based on the piston, and adjusts the eccentric cam rotation position using an oil control valve.