For treating mode conversion caused by discontinuity in multi-conductor transmission line with circuit analysis, we have proposed a modal equivalent-circuit model with mode-conversion sources. The approach takes an advantage in less calculation sources and countermeasure consideration compared with full-wave simulation. A mode-decomposition technique was applied with an imbalance factor, that is, the current division factor in our study, of the transmission line. In the modal circuit analysis we proposed, mode conversion is expressed by the controlled sources of which magnitude is proportional to the difference between the current division factors of the adjacent transmission lines. In this paper, we focused on the modal transfer power and derived the mathematical expressions of the mode conversion using the modal characteristic impedance as well as the current division factor. For validating the derived equations, in addition, the comparison with full-wave simulation was carried out and a good agreement was confirmed.