In this study, we investigated dewetting process of polystyrene and poly (vinyl methyl ether) blend thin films for the in-plane structure using light scattering (LS) and optical microscope (OM) as well as atomic force microscopy (AFM). The density profile perpendicular to the film surface was measured with neutron reflectivity (NR). In our previous work, we performed LS measurements and OM observations on the 98 nm film after temperature jump into two phase region. It was found that the time evolution of the scattering intensity was well described by the kinetics of spinodal decomposition type phase separation and we confirmed in the OM observation that the dewetting proceeded during the measurements, meaning that the kinetics of the dewetting is a spinodal decomposition type one in the thin film. In this work we found from NR results that concentration fluctuations occur normal to the surface before dewetting, which may induce the dewetting. The details will be discussed at the poster session.