Reduction of structure-borne sound is an important issue for industry mechanical products. This paper presents a new idea for controlling structural intensity (SI) on panel structure derived from a cancellation of SI due to mode pairs of multiple roots. At first the modal formulation of SI on a flat plate is summarized. Next we discuss the change of SI on a flat plate by attaching a reinforcement beam cross the plate in one direction. It is shown that this attachment give multiple natural frequencies, and their mode pairs and the pairs of modal components of SI are in same phase in the area of half of the plate and in anti-phase in that of the other half. Then the modes and modal components of SI can be said to be canceled by each modal pairs in the area of half on the plate. Based on this cancellation due to modal pairs, a new method for reducing transmitted power to a given area on the plate has been derived and proposed by attaching a beam on the plate for providing partial symmetry on the plate. It is demonstrated that the partial attachment of a beam on the plate based on the proposed method can reduce the transmitted power to the given area by the cancellation due to the pairs of SI modal components.