We have studied the structure of styrene-butadiene rubber with anomalous small-angle X-ray scattering (ASAXS) at sulfur K-edge. The use of ASAXS at sulfur K-edge selectively changes the scattering contrast relating to sulfur, thereby leading to the structural study of low-contrast samples that would otherwise be difficult to be conducted. The energy range around the K-edge of sulfur has not been applied to scattering study, owing to mainly the absence of beamline and detector appropriate for the energy range. We have developed ASAXS setting with a transmission geometry at BL27SU, SPring-8 and have successfully obtained the scattering information relating to inhomogeneous distribution of sulfur in styrene-butadiene rubber. A two-phase model is applied for elucidating a coarse-grained view of the inhomogeneous structure. The results indicate that inhomogeneous distribution of polymer and sulfur is closely related and the dense part of polymer corresponds to the part being sparse of sulfur. This study opens up the possibility to elucidate a complex structure of macromolecular systems with a low scattering contrast. (C) 2016 Elsevier Ltd. All rights reserved.