We propose elastic neutron powder diffraction experiments on perovskite antimonates (Ba,K)SbO3. Main-group oxide perovskites have attracted intense research interest due to their novel physical properties including high-Tc superconductivity and oxygen hole states. Recently, we have synthesized (Ba,K)SbO3 for the first time, and found that the undoped compound BaSbO3 may possess a characteristic structural distortion due to the presence of oxygen holes. High-resolution diffraction experiments at WISH will enable us to precisely determine the crystal structure of (Ba,K)SbO3. Our results will be useful to understand the important role of oxygen holes and its link to high-Tc superconductivity among the main-group oxide perovskite families, and can be applied to a design principle for new oxide superconductors.