The reduction of main hull vibration of ships by a semiactive dynamic absorber is investigated. The dynamic absorber system includes a moving mass, support springs, dynamic dampers and a control system. Only small electrical power supply is needed for the valve control of the damper and the operation of the control system. In this investigation, the operation theory of the dynamic absorber is first described. Then, a suboptimal control law for the absorber is derived based on the optimal theory. The numerical simulation results show that the dynamic absorber achieves better efficiency in hull vibration reduction than the passive type absorber during critical periodical excitation from the propeller. The vibration caused by multi-frequency excitation can also be suppressed by the dynamic absorber.