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In a differential transmission line, a large common-mode radiation is excited due to asymmetry. To suppress the radiation, the differential line must be designed electrically symmetric. In this paper, the imbalance difference model, which was proposed by the authors for estimation of common-mode radiation, is extended to apply the differential signaling system. The authors focus on two pairs of differential transmission lines with asymmetric property, which consists of an adjacent return plane and signal lines which are placed close to an edge of the return plane. The authors define five transmission modes; two normal modes, two primary common modes and a secondary common mode. In these transmission modes, the secondary common mode radiation is dominant, and the authors evaluate the radiation using the imbalance difference model. To reduce the common-mode radiation, placing a guard trace which has the same potential as that of the return plane, we can control the imbalance and reduce common-mode radiation even the transmission line has asymmetry. The reduction of common-mode radiation can be estimated quantitatively by calculation of the imbalance of the transmission line.