The electromagnetic topology and the BLT equation has been used as useful techniques to analyze coupling effects of huge devices. But in the case of systems including complex parts, applying the BLT equation can be difficult to manifest the complex parts with analytic solutions. To resolve this problem, a numerical method can be used to parts of the whole system in advance. In this paper, a microstrip line filter is analyzed using the BLT equation combined with numerical solutions. Consequently, achieved graphs from the BLT equation show good agreements with graphs obtained using a numerical method only.