This study investigates the influence of in-plane vibration on the construction of statistical energy analysis (SEA) models of flexural vibration in plate structures. A simple system is used that consists of two flat plates connected in an L-shaped configuration. For two SEA models, the loss factors related to flexural vibration are calculated by the finite element method, both with and without the in-plane vibration being taken into consideration. Differences occur in the flexural vibration loss factors in the frequency bands corresponding to the natural frequencies of the in-plane vibration in the unconnected plates. The subsystem energy, averaged over frequency and space, also shows the influence of in-plane vibration. As a result, a new method is developed to evaluate the effects of in-plane vibration on constructed SEA models of flexural vibrations. It is demonstrated that the method is useful in identifying the frequency bands affected by in-plane vibration and in determining under- or over-estimation of the loss factors.