Parallel processing of large-scale electromagnetic field simulations based on the diakoptics is a key to increase the complexity of computational targets. The time-domain discrete Green's function is usable as a means of modeling the torn regions. In this paper, a discrete Green's function of a unit-cell of Yee's FDTD lattice is introduced and a technique for calculating the Green's function is presented. The technique can be extended to the characterization of arbitrarily divided regions, which contributes to increasing the flexibility of subdivision of the whole structure.