This paper presents a method of vibration control for a beam carrying a mass at its tip subjected to earthquakes. A vibration isolation mechanism consisting of a gear train for the beam is presented. Theoretical analysis for the beam is developed, and to validate the method and analysis, experimental tests are carried out for a model of the present mechanism. It is clarified that the vibration displacements and the moments in the beam are suppressed significantly in comparison with a general beam without the mechanism. A method of optimal design has also been presented, and numerical calculations have been carried out for the beam with actual sizes. In the present beam, energies for controlling vibration are not required, because the earthquake force is utilized as a control force. Therefore, the structure using the beam has advantages as compared with the structure having an active vibration control system.