Acoustic characteristics in an industrial gas-turbine combustor are numerically investigated by a linear acoustic analysis. Spatially non-homogeneous temperature field in the combustor is considered in the numerical calculation and the characteristics are analyzed in view of acoustic instability. Acoustic analyses are conducted in the combustors without and with acoustic resonator, which is one of the acoustic-damping devices or combustion stabilization devices. It has been reported that severe pressure fluctuation frequently occurs in the adopted combustor, and the measured signal of pressure oscillation is compared with the acoustic-pressure response from the numerical calculation. The numerical results are in good agreement with the measurement data. In this regard. the phenomenon of pressure fluctuation in the combustor could be caused by acoustic instability. From the numerical results for the combustor with present acoustic resonators installed, the acoustic effects of the resonators are analyzed in the viewpoints of both the frequency tuning and the damping capacity. It is found that the resonators with present specifications are not optimized and thus, the improved specification or design is required.