[en] The crystal structure of BaFe₂S₃ is determined up to ∼12 GPa by means of x-ray powder diffraction experiments using synchrotron radiation at room temperature. BaFe₂S₃ is the first superconductor in iron-based ladder compounds, showing the critical temperature at T _c = 24 K under 11.6 GPa. The superconductivity emerges on the application of pressure after suppressing an antiferromagnetic Mott insulating state. The atomic positions in the superconducting state remain hitherto unknown experimentally. We found significant changes of structural parameters: shrinkage of the Fe-ladder unit and decreasing height of the S atom position from the Fe plane. Moreover, a reduction of the S–S distance between the ladders was observed under pressure. The pressure dependence of the structural parameters is strikingly different from the prediction of the band calculation. (paper)