There have been many reports on the design of active vibration control systems. Most of them have optimized control systems based on the LQ optimal regulator or H_∞ control theory. However, there are no answers to the problem whether given passive parameters of plants (e. g. , spring constant on damping coefficient) are 'suitable' for the designed controller. In this paper, we consider structure and control simultaneous optimization for an active vibration control system. To provide solutions, several methods have been proposed which are based on tuning the design paramethers existing in the structure and controller. Most of the control laws are those of standard LQ optimal requlators which require full states measurements and estimate performance of a closed-loop control systems in the time domain. From the viewpoint of active vibration control, it is natural to discuss the performance of the control system in the frequency domain. Therefore, we adopt the frequency shaped LQG control law in simultaneous optimization design, which enables output feedback control and frequency shaping.