The reduction of superstructure vibration is essential for securing comfortable accomodation space and for preventing damage of delicate equipment in ships. Superstructure vibration is divided into three categories. The first is the local vibration where decks and walls vibrate locally. The second is the fore-aft vibration of the superstructure where the whole superstructure vibrates in the fore and aft direction at its resonant frequency. And the third is the vibration which is caused by hull girder vibrations. The prevention of fore-aft vibration is the most important, because the cure is difficult and expensive in many cases. The authors have developed a dynamic vibration absorber of which the natural frequency can be controlled by adjusting the location of the mass. The reduction of the superstructure fore-aft vibration by means of the dynamic vibration absorber were studied, and are presented in this paper. Scale model tests and full scale tests were performed. The superstructure and the dynamic vibration absorber were modelled into two degrees of freedom systems with the help of FEM analyses. The experimental results were compared with the results calculated by the simple formula of two degrees of freedom systems. They generally showed good correlation. The vibration of the superstructure of a 40, 000 DWT cargo ship was reduced to one forth of the original vibration level by using the dynamic vibration absorber with a 800 kg weight. The dynamic vibration absorber has the following advantages : (1) The installation requires little change in the vibrating structure, so the installation is economical. (2) The dynamic vibration absorber itself generates no vibration, nor noise, because it requires no power sources. (3) The effect of vibration reduction is constant and reliable.