The slit pattern formed on the return plane of printed circuit boards (PCBs) acts like a passive element and a defected ground structure (DGS) is one of them. In this paper, we propose an equivalent circuit model with a transmission line model for use in DGS with a filter characteristics. The characteristics of DGS are easily varied with the slit pattern. Thus, DGS is expected to be used for various applications such as a common-mode filter in differential signaling systems. Since a design method has yet to be developed, however, we need to establish a design method for the slit pattern. Some equivalent circuit models have been used, but the models that consist of lumped elements require a full-wave simulation. In addition, the values of the lumped elements do not relate to the physical parameters. Therefore it is not useful for designing filters. In contrast, the equivalent circuit model we propose in this paper will have a great contribution to designing filters with optimum performances and fit for size reduction on PCBs because the transmission line model relates to the physical parameters. As a result, by comparing the transmission characteristics calculated with both a full-wave simulator and a circuit simulator with the proposed equivalent circuit model, the first stop-band width calculated by using the circuit simulator was in agreement with the full-wave simulator.