In grounding an STP cable, mode conversion occurs depending on how to ground it. For investigating the mode conversion in a four-conductor transmission-line system that consists of three conductors and the system ground such as the STP cable above the system ground, as shown in Fig. I. we have developed modal equivalent circuit expressed by an imbalance factor of transmission line, a current division factor, in the same way as we developed the one in a three-conductor transmission-line system. The modal equivalent circuit consists of three modal equivalent circuits of normal mode (Fig. 2), primary common mode (Fig. 3), and secondary common mode(Fig. 4). In this paper, mode conversion between normal mode and primary common mode was evaluated using mixed-mode S parameters and the measured data validated the modal equivalent circuit. As a result, it was found that mode conversion depends on the imbalance factor and it was examined how to ground for suppressing mode conversion.