The space formed by the ground and ionosphere is known to act as a resonator for extremely low frequency (ELF) waves. Lightning discharges trigger this global resonance, which is known as Schumann resonance. Even though the inhomogeneity (like day-night asymmetry, local perturbation, etc.) is important for such a subionospheric ELF propagation, the previous analyses have been always made by some approximations because the problem is too complicated to be analyzed by any exact full-wave analysis. This paper presents the first application of the conventional numerical FDTD method to such a subionospheric ELF wave propagation, in which any kind of inhomogeneity can be included in this analysis. However, the present paper is intended to demonstrate the workability of this method only for a uniform waveguide (without day-night asymmetry), by comparing the results from this method with those by the corresponding analytical method.