A variable cam phaser for an automobile engine including: a drive rotor driven by the crankshaft of the engine; a control rotor; a camshaft coaxial with the drive rotor and adapted to rotatably support the drive rotor; a torque means for providing the control rotor with a torque for rotating the control rotor relative to the drive rotor; a phase angle varying mechanism for varying the relative phase angle between the drive rotor and the camshaft in accord with the relative rotation of the control rotor relative to the drive rotor; and a self-locking mechanism mounted in the phase varying mechanism for preventing a phase change from occurring between the drive rotor and the camshaft caused by an unexpected cam torque appearing on the camshaft, the variable cam phaser characterized in that the self-locking mechanism comprises: an eccentric circular cam integral with the camshaft; and a lock plate having a support groove for supporting, at positions further offset from the camshaft axis in the direction (referred to as eccentric direction) from the camshaft axis towards the cam center of the eccentric circular cam, the periphery of the eccentric circular cam from both sides thereof, a coupling mechanism for transmitting the relative rotational torque from the control rotor to the eccentric circular cam, and a cylindrical body formed integral with the drive rotor and circumscribing the periphery of the lock plate. a drive rotor driven by the crankshaft of the engine; a control rotor; a camshaft coaxial with the drive rotor and adapted to rotatably support the drive rotor; a torque means for providing the control rotor with a torque for rotating the control rotor relative to the drive rotor; a phase angle varying mechanism for varying the relative phase angle between the drive rotor and the camshaft in accord with the relative rotation of the control rotor relative to the drive rotor; and a self-locking mechanism mounted in the phase varying mechanism for preventing a phase change from occurring between the drive rotor and the camshaft caused by an unexpected cam torque appearing on the camshaft, the variable cam phaser characterized in that the self-locking mechanism comprises: an eccentric circular cam integral with the camshaft; and a lock plate having a support groove for supporting, at positions further offset from the camshaft axis in the direction (referred to as eccentric direction) from the camshaft axis towards the cam center of the eccentric circular cam, the periphery of the eccentric circular cam from both sides thereof, a coupling mechanism for transmitting the relative rotational torque from the control rotor to the eccentric circular cam, and a cylindrical body formed integral with the drive rotor and circumscribing the periphery of the lock plate. an eccentric circular cam integral with the camshaft; and a lock plate having a support groove for supporting, at positions further offset from the camshaft axis in the direction (referred to as eccentric direction) from the camshaft axis towards the cam center of the eccentric circular cam, the periphery of the eccentric circular cam from both sides thereof, a coupling mechanism for transmitting the relative rotational torque from the control rotor to the eccentric circular cam, and a cylindrical body formed integral with the drive rotor and circumscribing the periphery of the lock plate.