In order to predict sound fields in rooms with membrane materials, a new method using limp membrane elements in acoustical FEM analysis is developed, which is based on a limp membrane model. This method brings about a simple and easy treatment for membranes and a reduction in computational storage and time. The method is validated through its application to an ideal sound field solvable theoretically; and subsequently, it is applied to two kinds of analyses on a model room. The results computed are compared to those experimented, where good agreement can be seen. Moreover, the superiority of the method over the ordinary method assuming local reaction is confirmed in prediction accuracy.