We propose elastic neutron powder diffraction experiments on perovkite bismuthates (Ca,K)BiO3. Known as one of the earliest high-Tc oxide superconductors, bismuthates have provided a model system for studying novel superconductivity intertwined with a lattice structure. Recently, we have synthesized (Ca,K)BiO3 for the first time, and found that superconducting Tc was remarkably decreased (~8 K) compared to the sister compounds (Ba,K)BiO3 (34 K) and (Sr,K)BiO3 (13 K). High-resolution diffraction experiments at HRPD will enable us to precisely determine the crystal structure of (Ca,K)BiO3, in which large octahedral tilt distortions are expected due to the smaller Ca ions. Our results will be useful to understand the origin of superconductivity and its link to structural distortions among the bismuthate families, and can be applied to a design principle for new oxide superconductors.