As a method of calculating impulse responses in a room, the application of the finite-difference method (FDM) has been investigated. In this paper, the principle of the numerical calculation is introduced first, and its applications for obtaining impulse responses in several two-dimensional spaces with typical shapes are presented. In order to see the validity of the calculation by FDM, the calculated results are compared to the results obtained in scale model experiments. Next, to apply the FDM calculation method to rooms with absorbing boundaries, two physical models based on the mechanical substitution model and the Rayleigh model are proposed. In order to examine these models, the results obtained by numerical calculation and by experiment are compared.