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This article presents a numerically stable explicit time-domain finite element method (TD-FEM) for room acoustic simulation using an equivalent impedance model. Performance over a conventional explicit TD-FEM is demonstrated using numerical experiments. Both explicit TD-FEMs were based on simultaneous first-order ordinary differential equations. The conventional method has a stability problem derived from backward difference approximation of time derivative of sound pressure related to a dissipation term for treating absorption at a boundary. The numerically stable explicit TD-FEM avoids the stability problem through the combined use of first-order forward and backward difference approximation. A completely explicit algorithm is obtainable using a lumped dissipation matrix. Numerical results demonstrate that the proposed explicit TD-FEM is stable for changing impedance values at boundary surfaces. In addition, numerical dissipation and dispersion error analyses revealed that both explicit TD-FEMs have small dissipation error. Moreover, the proposed method has slightly lower accuracy than the conventional explicit TD- FEM. However, the proposed method is computationally more efficient than the conventional method for acoustics simulation of rooms with smaller impedance values at boundaries. [ABSTRACT FROM AUTHOR]