Structure-borne sound is an important issue in the field of architectural acoustics. In order to predict such transmission characteristics of vibration, prediction methods using wave based numerical analysis have been proposed. Nicole and Kessissoglou proposed a method for predicting the power transmission through a simple L-shaped structure composed of two plates using a combination of the modal method and the traveling wave method, which results in an exact solution. With this scheme, the vibration characteristics can be solved. However, it is difficult to apply it to structures with a complex shape, owing to the feature of the exact solution. The physical parameters of the displacement, the rotation angle, and the bending moment, which act in the vicinity of the edge of the plate, are made to be related to those at the edge of the other plate. In addition to these conditions, another condition that the shear force acting on the edge part of one plate is related to the axial force acting on the edge of the other plate is also considered.