AbstractAn integral equation method is used to analyze the scattering of the incident guided mode at the gap between two dielectric slab waveguides with arbitrarily shaped ends. As a typical example, a two‐dimensional model of the coupling gap, which occurs in the preheating stage of the fusion splicing of optical fibers is treated. The possibility is studied for indirect understanding of the situation of the coupling location from the measured values of the reflected light, transmitted light, etc. A detailed study is done for the correlation of the reflected, transmitted, radiated power and radiation patterns with the gap width and the end shape (functions of the preheating time). The reflected, transmitted, and radiated power change quickly within a short period but without significant characteristics. However, their envelopes indicate characteristic tendencies. The maximum of the reflected power and the difference between the maximum and minimum of the transmitted power decrease monotonically. The backward, rather than forward, radiation pattern varies more sensitively to the change of the end shap