In this work, we investigate the formation of interference patterns appearing in s-NSOM results. A single nanoslit is used to demonstrate the mechanism of formation of these interference patterns experimentaly: the interaction between the in-plane component of the incident light and SPP launched by the nanoslit. This is in contrast to some other explanations that the SPP is launched from the NSOM probe. We also use an analytical model and numerical simulations to compute the formation of interference patters. This study will help to understand s-NSOM results from plasmonic nanostructures.