This paper investigates influence of in-plane vibration on building SEA model for flexural vibration. FEM calculation is carried out for the comparison of the SEA loss factors concerning flexural vibrations between with and without taking the in-plane vibrations into consideration. The test structure in this paper is a simple system which is composed of two plates connected in an L-shaped configuration. As a result, a new guideline for judging the influence of in-plane vibration in advance is introduced. The natural frequency analysis for uncoupled subsystem is used to understand the frequency range affected by the in-plane vibration. The degree of influence for the in-plane vibration can be predicted from the proposed estimation method that is derived by the flexural vibration results and wave theory.