Generally, Mechanical structures installed in nuclear power plants are designed statically in elastic region. But from the viewpoint of cost, this method includes a lot of margin and it isn't an efficient method. Recently, Energy Balance Method is watched as a new design and evaluation method. In this report, Extended Energy Balance Equations for Multi-degree-of-freedom system are constructed. Calculation and Experiment results of these equations indicate that accumulation of energy is as little as single-degree-of-freedom system in Hi-Frequency region.