It has already passed about 40 years since small-angle neutron scattering (SANS) technique was applied in structural analyses. SANS is now one of common tools in structure investigations of soft materials. In this article, we briefly describe the basis of neutron scattering, SANS instruments, and theories of SANS. Particularly, we focus on cross-link inhomogeneities and propose two methods. One is the visualization-by-swelling method and the other the contrast variation method. Then, we show several topics in structure investigations of elastomers, such as, natural rubbers and isoprene rubbers cross-linked by sulfur and peroxides.