Abstract: A vibration damper for damping roll vibration of a power unit mounted on a vehicle body. The vibration damper is composed of a disc plate fixedly connected to the power unit in a manner that the axis of the disc plate is generally parallel with a roll axis of the power unit. The disc plate is sealingly disposed in a chamber formed in a generally circular casing which is coaxial with the disc plate. A viscous fluid is filled in a space defined between the disc plate and inner surface of the casing. The casing is formed of a plastic which has a higher linear expansion coefficient than a metal forming the disc plate, thereby allowing the space filled with the viscous fluid to expand along with thermal expansion of the viscous fluid.