Semi-active suspension is one of many effective devices to improve vehicle stability, controllability and riding comfort. A practical means to realize semi-active suspension is to vary the damping force of the shock absorber. In this paper, we propose a new type of shock absorber using a piezoelectric sensor and actuator. The piezoelectric sensor and actuator are built into the piston rod which is a part of the shock absorber. The piezoelectric element provides a fast response and a high actuation force. We used the piezoelectric element in shock absorbers in order to take advantage of these two features. High level compatibility between stability, controllability and riding comfort is expected, since damping force changes very quickly using this new type of shock absorber.In this paper, several topics are discussed. First, a general description of the damping force control system with simple configuration is explained. Second, features of the piezoelectric sensor and interface circuits used to detect road surface conditions are shown. Third, features of the piezoelectric actuator and displacement magnification chamber are discussed. Fourth, reduction of noise generated at the moment of valve opening is discussed. The noise is generated by a rapid pressure change when the damping force changes quickly. The methods to realize quick damping force change with low noise are discussed. Effective reduction of the noise is shown to be influenced by the shape of a valve through experiments.