Non-uniformity in cross-linked rubber is one of important factors to control mechanical properties in rubber. This review presents an analysis of the non-uniformity by using small angle neutron scattering (SANS) method. In the method, the rubber is swollen by solvent and the non-uniformity as well as the network structure can be observed by SANS. The observation of smaller wavenumber region with Focusing SANS has clarified that another non-uniformity exists in the order of 100 nm in addition to non-uniformity found previously by Ikeda et al.