A damping coefficient control device, for use in a vibration damper of the type which is disposed between a vibration source and a vibration receiving section to be isolated from the vibration of the vibration source and which has a plurality of small liquid chambers charged with an electrorheologic fluid and communicated through an orifice, includes electrodes disposed in the orifice to receive a voltage which is suitably controlled so as to cause a change in the viscosity of the electrorheologic fluid, thereby controlling the damping coefficient provided by the fluid. The damping coefficient control device has sensors for sensing the velocities of the vibration source and the vibration receiving section. When the direction of the velocity of the vibration receiving section is the same as the direction of the relative velocity of the vibration receiving section relative to the vibration source, the voltage applied to the electrodes is controlled such as to increase the damping coefficient provided by the electrorheologic fluid, whereas, when these directions are different, the voltage is controlled such as to decrease the damping coefficient. This arrangement enables the vibrations of different frequencies to be damped without any risk that resonance will occur.