The estimation theory of sound power which has been established based on the analytical modal analysis is extended to application based on FEM modal analysis. We tried to estimate the sound power radiated from a semisphere, which is a simple model of the lid of an automobile transmission case and is difficult to analyze the natural modes by analytical modal analysis. Next, the mesh size of the FEM model, with which the estimation of sound power will converge within the accuracy of ±0.3 dB, was investigated using some FEM models with various mesh sizes. As a result, the estimated sound power coincided with the measured values with an accuracy sufficient for practical usage, and converged within the accuracy stated above when the maximum mesh size was smaller than one-sixth the wavelength of the highest frequency component. These results were confirmed by applying them to similar structures, such as derby hat and silkhat structures.