In general, vibrations transmitted to mounting systems contain both low frequency vibrations and high frequency ones. Mounting systems cannot have characteristics such as to cut off both low frequency vibrations and high frequency ones. If a special device cutting off high frequency vibrations is attached to a mounting system, the characteristics of the system can be easily fitted for only low frequency vibrations. This paper shows the development of a mounting system having a small-sized oscillating actuator built-in. This system is composed of an oscillating actuator newly developed, the detection of output plate vibration acceleration, and an actuator controller. An oscillating actuator is set on an input plate of the mounting system and excites an intermediate plate to make the output plate vibration nearly zero. The actuator controller digitally processes vibration acceleration data picked up on the output plate and makes up output data for the actuator, checking the vibration characteristics and selecting a suitable phase shift to reduce the vibration. It is shown that more than 70 % of higt frequency vibration can be eliminated by this mounting system.